In the 50s and 60s of the last century there was a movement of the so-called God-is-dead theology, also known as theothanatology, theos (God) and thanatos (death).

I think the best and most plausible theory that God is really and literally dead can be reconstructed from the pessimistic philosophy of the German Philipp Mainländer (1841 - 1876).

This is what I will present here in a bundled way.

Mainländer famously makes a bold notorious claim:

“God is dead and his death was the life of the world.”

Mainländer’s specific reasoning for this statement is as follows:

“(1) God willed (his own) non-being.

(2) God’s immediate passage into non-being was impeded by own being. [Had God’s will directly achieved its end, then worldless non-being would presently prevail; and since nothing outside God can act on him, only God’s own being could have impeded his will.]

(3) It was consequently necessary for God’s being to disintegrate into multeity, a world in which each individual being strives to achieve non-being. [Only the finitization of God’s being will allow the end of non-being to be achieved.]

(4) Individual worldly beings hinder one another’s striving and, in so doing, weaken their degree of force.

(5) God’s entire being underwent transformation into a determinate sum total of forces.

(6) The world as a whole or universe has one end, non-being, which it will achieve through the continual diminution of the sum of forces which compose it.

(7) Each individual being will be brought in the course of its development, by virtue of the dissipation of its force, to a point where its striving to non-being is fulfilled.” (Translation and comments by Sebastian Gardner in “The Oxford Handbook of Schopenhauer”)

This reasoning can be demonstrated in deductions, which Mainländer himself does not make, but they can be reconstructed from his philosophy.

All of the following premises could, in my opinion, be supported with very good and, above all, reasonable arguments, even if one would not necessarily agree with them.

Also important to keep in mind: The deductions depend on each other.

The first deduction:

A 1. The universe had an absolute beginning a finite time ago.

A 2. Only through an act originating from God could the beginning of the universe have been set.

B 1. God can produce something only out of his own substance (contra creatio ex nihilo et non se Deo, that is, creation from nothing and not from God).

B 2. In the case of the coming into being of our universe this would have to be understood as transformation of something divinely transcendent into something worldly immanent.

C 1. God’s wisdom strictly forbids coexisting with or alongside a creation in which everything that happens happens necessarily and without real alternatives.

C 2. God can never create anything else than that whose activity from the outset will always lead only to a very specific and certain outcome, necessarily and inevitably so, due to Efficient Causes (determinism) and/or Final Causes (teleologism), thus according to The Principle of Sufficient Reason.

D Therefore, God has completely transformed himself into the universe.

The second deduction:

God turned into either (x) a temporally limited universe or (y) a temporally infinite and everlasting one.

1.1 If the latter (y) is the case, God has transformed into something that is inferior to his original state in terms of mode of existence. Even if God should turn into a timeless eternal universe, this universe would be ontologically less perfect compared to his primordial oneness.

i) However, God’s most perfect wisdom forbids irrevocably entering (irreversibly) an inferior existence.

1.2 If the former (x) is given, then at some point the temporally limited universe either returns into the exact original state of God, which has gained nothing and lost nothing by the process, or it ends in absolute nothingness.

ii) However, God does not do anything superfluous or pointless.

2. Therefore, the following applies: “God’s entire being underwent transformation into a determinate sum total of forces (a Kraftsumme).” And: “The world as a whole or universe has one end, non-being, which it will achieve through the continual diminution of the sum of forces which compose it.” (Mainländer, translated by Sebastian Gardner)

The third deduction:

I. God could not immediately erase himself from existence.

II. The immediate erasure of his own existence, an existence which is in a certain way identical with his omnipotence, presupposed this omnipotence. In other words, his omnipotence could theoretically wipe out everything created without delay, except itself, because its immediate annihilation would require or necessitate its complete existence at the same time (concurrently).

III. Therefore, God had no choice but to become a slowly but steadily disintegrating and waning world that, once gone, leaves absolutely nothing behind, in the truest sense of the word.

The fourth deduction:

I. God enjoys being the most perfect and blissful being.

II. Thus, the following is true: “If the Eternal be conceived as in complete and perfect bliss, happily static and statically happy, there is no reason in logic or in life why he should ever be moved to engage in creation.” (Brasnett, Bertrand R. – The Suffering of the Impassible God)

III. God enjoys absolute freedom to remain in existence or not to be at all (Real Libertarian Free Will).

IV. If he should ever be moved to engage in creation, it would be for the reason of ceasing to be.

V. There is creation, that is, a world as the sum of a multitude of individuals.

VI. In addition, the following applies: The difference between monotheism and pantheism is “only an apparent one, a difference on the surface.” (Mainländer)

“They have one common root: absolute realism and both have exactly the same crown: the dead individual which lies in the hands of an almighty God[:]” (Mainländer)

"When the individual acts, his action will be not his own but only the single universal substance [God] acting through him." (Frederick C. Beiser - Weltschmerz)

“A basic unity in the world [pantheism] is incompatible with the always and at every movement obtruding fact of inner and outer experience, the real individuality.” (Mainländer)

VII. I experience myself not only as an individual, but also as a very alive one.

VIII. God “cannot have chosen to remain in being or to merely alter his manner of being, else no world would have come into existence.” (Sebastian Gardner commenting on Mainländer’s sentence: God willed (his own) non-being.)

IX. Instead of dead individuals and a living God, there are living individuals and a dead God.

Some comments:

Mainländer identifies the monotheistic God with a cat that has created a mouse, i.e. a determinate living being, in order to play sadistically with it. A truly wise God would possibly not want to take over the role of a cat, whose mouse-creation has no real freedom and reacts only necessarily to His actions. In fact, the Bible really seems to uphold a feline image of God, with some mice being spared, even rewarded:

Jeremia 10,23: I know, O Lord, that the way of man is not in himself, that it is not in man who walks to direct his steps.

Proverbs 21,1: The king’s heart is in the hand of the LORD, like the rivers of water; He turns it wherever He wishes.

Exodus 4,21: The Lord said to Moses, “When you return to Egypt, see that you perform before Pharaoh all the wonders I have given you the power to do. But I will harden his heart so that he will not let the people go.

Romans 8,28: And we know that in all things God works for the good of those who love him, who have been called according to his purpose.
Romans 8,29: For those God foreknew he also predestined to be conformed to the image of his Son, that he might be the firstborn among many brothers and sisters.
Romans 8,30: And those he predestined, he also called; those he called, he also justified; those he justified, he also glorified.

Romans 9, 15: For he says to Moses, I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.
Romans 9, 16: So then it depends not on human will or exertion, but on God, who has mercy.
Romans 9, 18: So then he has mercy on whomever he wills, and he hardens whomever he wills.

The idea that God cannot possibly create free beings goes back to Schopenhauer:

“Everything that is also is something, has an essence, a constitution, a character; it must be active, must act (which means to be active according to motives) when the external occasions arise that call forth its individual manifestations. The source of its existence is also the source of its What, its constitution, its essence, since both differ conceptually, but in reality cannot be separated. However, what has an essence, that is, a nature, a character, a constitution, can only be active in accordance with it and not in any other way; merely the point in time and the particular form and constitution of the individual actions are each time determined by the occurring motives. That the creator created human beings free implies an impossibility, namely that he endowed them with an existence without essence, thus had given them existence merely in the abstract by leaving it up to them what they wanted to exist as.” (§13. Some further elucidations on the Kantian philosophy Parerga and Paralipomena Short Philosophical Essays Volume 1 Arthur Schopenhauer)

“The truth, however, is that being free and being created are two qualities that cancel and thus contradict one another. So the claim that God has created beings and at the same time given them freedom of the will really means that he created them and at the same time did not create them. For acting follows from being,a that is, the effects, or actions, of any possible thing can never be anything else but the consequence of its constitution,b which itself is known only through the effects. Therefore, in order to be free in the sense here demanded, a being would have to have no constitution at all, in other words, be nothing at all, thus be and at the same time not be. For what is must be something; an existence without essence cannot even be thought. If a being is created, then it is created in the way it is constituted; thus it is created badly if it is constituted badly, and constituted badly if it acts badly, meaning, having bad effects.” (§9. Scotus Erigena Parerga and Paralipomena Short Philosophical Essays Volume 1 Arthur Schopenhauer)

There is also a certain similarity of Mainländer to the mystic Jakob Böhme:

“The Supreme does not create out of nothing. Ex nihilo nihil fit—out of nothing nothing comes. He produces from His Own eternal nature and eternal wisdom, wherein all things dwell in a latent condition, all contrasts exist in a hidden or non-manifest state. When the Verbum Fiat, or Spoken Word, goes forth, these hidden principles — the qualities, forms, colours, powers, etc. — arise in a manifestation of glorious celestial orders in a universe of angelic beings whose life is light, joy, and peace.” (W. P. SWAINSON – JACOB BOEHME. THE TEUTONIC PHILOSOPHER)

Western philosophy has made the mistake of thinking that whatever exists perfectly necessarily wants to exist or to remain in existence. But it is not a logical contradiction, because it concerns only a question of value, that the perfect being can choose non-being in spite of its perfection. God may very well come to the conclusion that non-being is better than any form of being, even the divine one.

Buddhism, now culturally very influential, is definitely in line with Mainländer’s thinking, unlike Hinduism:

“There was a definite shift of values when Buddhism emerged from Hinduism. Even though both groups retained the concept of Nirvana, the definition of Nirvana shifted from being merged with ultimate reality to extinction.” (Yancey, George; Quosigk, Ashlee – One Faith No Longer)

Even Christianity, in certain respects and in a limited way, namely with regard to the voluntary death on the cross of the Son of God, does not seem to be as far away from Mainländer as some might think:

“[John] Donne […] wrote Biathanatos, a defense of outright suicide in which Jesus himself is chief among the exemplary suicides of the past. Biathanatos—so daring in its day that it could be published only after Donne’s death—is a tour de force of authentic intellectual passion. A fiercely brilliant scholar who once confessed a “sickely inclination” to become a biathanatos (that is, a suicide: the Greek word means “one dead by violence, especially self-inflicted”), Donne was paradoxically strengthened by his pathology to trace Christian martyrdom to its source in the suicide of God Incarnate. The ambiguity of the question resides in the fact that Christ is a suicide by metaphysical definition, whether or not he is a suicide in some more ordinary sense of the word. That is, if Jesus is God Incarnate, then no one can have taken his life away from him against his wishes. His suicide is, in this regard, as deeply built into the Christian story as the doctrine of the Incarnation. Thus, for Thomas Aquinas, Jesus was the cause of his own death as truly as a man who declines to close a window during a rainstorm is the cause of his own drenching. Thomas strongly implies, moreover, that those who actually killed Jesus, or conspired to kill him, were less than fully responsible agents, that they were tools in the hand of God, a species of human rainstorm drenching God because God wished to be drenched. There is support for the latter view in the New Testament itself. From the cross, Jesus says of his executioners, “Father, forgive them, for they know not what they do” (Luke 23:34). Peter, preaching in the Temple after Jesus’ death, says, “Now I know, brothers, that neither you nor your leaders had any idea what you were really doing; but this was the way God carried out what he had foretold when he said through all his prophets that his Christ would suffer” (Acts 3:17–18). But granting that Jesus is a suicide at least in this unique sense, is he a suicide in any more ordinary sense? Can his death be linked with the despair that precedes “private” suicide? Or was the ignominious suicide of Judas, Jesus’ betrayer, added to the Gospel story precisely as a reminder that a chasm separates ordinary human suicide from the suicide of the God-man? Dauzat, building on the contemporary philosophical debate over suicide, wants to see an overlap such that what is said theologically about Christ’s suicide can bear philosophically on the discussion of suicide in general. Voluntary, self-inflicted death, he says, typically represents the rejection of a marred or strangled life in the name of “une vie dont on ne meurt pas,” “a life you don’t die of.”” (Jack Miles – Christ: a crisis in the life of God)

Cosmological proofs of God do not necessarily lead to a God who still exists:

“Even if valid, the first-cause argument is capable only of demonstrating the existence of a mysterious first cause in the distant past. It does not establish the present existence of the first cause. On the basis of this argument, there is no reason to assume that the first cause still exists — which cuts the ground from any attempt to demonstrate the truth of theism by this approach.” (George H. Smith – Atheism. The Case Against God)

“Indeed, why should God not be the originator and now no longer exist? After all, a mother causes a child but then dies.” (Peter Cole – Philosophy of Religion)

“This world […] is the production of old age and dotage in some superannuated deity; and ever since his death has run on at adventures, from the first impulse and active force which it received from him….” (David Hume – Dialogues concerning Natural Religion Part V)

Also a postulated necessity of the existence of God probably does not exclude the possibility of his self-annihilation:

“What about the necessary existence of God? I have already suggested that what is metaphysically necessary is God’s initial existence. I see no reason to hold that God necessarily continues to exist. That is, I hold God had the power to bring a universe into being and then cease to exist, while the universe went on. I do not believe that God has exercised that power, and if you hold that God never had it, so be it.” (Peter Forrest – Developmental Theism: From Pure Will to Unbounded Love)

“[T]he reasons given for believing that there is a necessary and simple being are only reasons for holding that, necessarily, at some time, there exists such a being. There is nothing incoherent in the idea that there was a first moment of Time, and that everything that was the case then was necessarily the case, including the existence of a simple being. That leaves open the possibility that this being might change or even cease to exist, contrary to classical theism.” (Peter Forrest – Developmental Theism: From Pure Will to Unbounded Love)

This depends on a certain conception of time:

“For Time, I take it, is characterized by the before/after relation between its parts. As it is, there is a succession of other moments. Brian Leftow has pointed out that if you are the only person at the counter, you are not a queue, and that Time is like a queue in that respect. But as soon as someone else comes along, there is a queue, and you are at the head of it (Leftow 2002). Likewise, if there are no other moments because God chooses to do nothing, then that moment is timeless. Yet if God acts, there is then at least one other moment, and so there is Time. If God chooses to create this universe, then the creative act is before now, and so God is not eternal.” (Peter Forrest – Developmental Theism: From Pure Will to Unbounded Love)

That the universe had a beginning is something that all the evidence of empirical and theoretical physics now overwhelmingly supports:

“The discovery that the universe is not static, but rather expanding, has profound philosophical and religious significance, because it suggested that our universe had a beginning. A beginning implies creation, and creation stirs emotions.” (Lawrence M. Krauss – A universe from nothing: why there is something rather than nothing)

And that one can say God for the cause of the universe is supported by the following argument:

“[W]e have in this case the origin of a temporal effect from a timeless cause. If the cause of the origin of the universe were an impersonal set of necessary and sufficient conditions, it would be impossible for the cause to exist without its effect. For if the necessary and sufficient conditions of the effect are timelessly given, then their effect must be given as well. The only way for the cause to be timeless [...] but for its effect to originate anew a finite time ago is for the cause to be a personal agent who freely chooses to bring about an effect without antecedent determining conditions.” (Philosophical Foundations for a Christian Worldview by J.P. Moreland William Lane Craig )

Mainländer’s God or Simple (Basic, Primal) Unity (the One) is a Pure Contingency. That is, it might no longer be, or it might be different. This must not be misunderstood. It is a contingency of whither or where to and not one of whence or where from. That is, across all possible worlds, Mainländer’s One would always be the absolute basic and starting condition. It is contingent in the sense that it can be “willfully and deliberately” different or not at all. Yet it itself has not been caused to exist and cannot disappear at random, because it is the logically simplest, but at the same time also the “mystically” richest thing one can think of. It is Pure, Simple, Undifferentiated, All-Powerful, Intellectual, Wise, Self-Aware, Creative Freedom (of Choice) to remain as it is or not to be, without any existential pressure to act, and therefore totally at ease, in peace and serenity.

For those who still believe in the impossibility of God’s irretrievable disappearance, should consider the following:

“God is whatever God is. I don’t think It is constrained by human interpretations of what it can or should be, can or should do.” (T Clark from The Philosophy Forum)

The advantages of Mainländer's conception is that one does not have to worry any more about a transcendence coexisting with the world, whose influences would be incomprehensible for us. With Mainländer there is only one reality, the pure comprehensible immanence.

Another advantage is that Mainlander's philosophy perfectly fits the physical law of entropy. And this law is still the generally accepted one in physics today.

The Christian doctrine of the Trinity is also reinterpreted accordingly by Mainländer:

"[...] Mainländer introduces his dramatic concept of the death of God (108). This primal unity, this single universal substance, has all the attributes of God: it is transcendent, infinite and omnipotent. But since it no longer exists, this God is dead. Yet its death was not in vain. From it came the existence of the world. And so Mainländer declares in prophetic vein: “God is dead and his death was the life of the world” (108). This is Mainländer’s atheistic interpretation of the Christian trinity, to which he devotes much attention in the second volume of Die Philosophie der Erlösung. “The father gives birth to the son”—Article 20 of the Nicene Creed—means that God (the father) sacrifices himself in creating the world (the son). God exists entirely in and through Christ, so that the death of Christ on the cross is really the death of God himself. With that divine death, Mainländer proclaims, the mystery of the universe, the riddle of the Sphinx, is finally resolved, because the transcendent God, the source of all mystery, also disappears." (Frederick C. Beiser - Weltschmerz)

And:

"The personal God of Christianity was in reality a concession to Jewish monotheism, as its doctrine of a future life was a concession to the insufficiently tamed Jewish vitality. "Esoteric Christianity" is atheistic; in a veiled form it teaches the doctrine of the selfannihilation of the godhead that existed "before the world": and the real reward of the Christian virtues is the "beatitude felt as contrast through reflection of not-being". This meaning of Christianity is developed in a remarkable essay on ''The Doctrine of the Trinity" (vol. ii. 190-232). As characteristic examples of Mainlander's interpretations of Christian theology, it may be mentioned that in his view "the Holy Ghost is the way of God to not-being," and is identical on the one hand with "the fate of the world," on the other hand with "the Christian virtues "by which that fate is directly accelerated ; while "Satan is the personified means to the end," "the wild struggle of individual wills"." (T. Whittaker - review. In: Mind. A quarterly review of Psychology and Philosophy. XI (1886))