In a recent thread I outlined three areas that might typify the thinking of an atheist intent on justifying their view. This seems to me to have some potential for further discussion.

The three areas to consider are firstly, showing problems with the arguments that purport to demonstrate that god exists, then pointing out inconsistencies in the notion of god that render him logically impossible, and finally rejecting the immorality that so often follows from theism.

I'll place posts outlining each, but first a few general notes. Here I'm looking at atheism as the belief that god does not exist. That's how it is generally defined, and places it in direct opposition to theism, the belief that god exists, and is contrary to the agnostic view that there is no good reason to think that god doesn't exist.

In a way this thread is an antidote to several threads started by theists on the forum seeking, perhaps duplicitously but certainly without success, positive arguments from atheists for their view.

Amongst professional philosophers and there associates theism is overwhelmingly rejected. In forums such as this, there are a small number of theists who are quite prolific. This might give an impression that theism, or other beliefs in God are prominent amongst amateurs, but it's more likely that those who don't believe in god just ignore the threads. Perhaps giving the discussion it's own place will entice a few from them.

Arguments for the existence of god

The basic argument is that if these were successful, there would be far more theist philosophers.

Logicians have given us a better understanding of necessity, the central theme of the cosmological and ontological arguments, than we have ever had. Those developments in logic have not lead towards wider acceptance of theism, but rather has shown further problems with the notion of a necessary being.

Teleological approaches have had some attention recently, having previously faded in the face of developments in our understanding of the logic of evolution. The catch here is that any purpose one chooses to consider may be imputed by us rather than observed independently. The important point here is that an atheist is not obliged to ignore the astonishing intricacy and beauty of the world around us, but can be whole heartedly amazed and yet not conclude that any of this implies the existence of a god.

Moral arguments seem to be little more than wishful thinking.

Far more can be said here, and doubtless will be both here and in other threads. But while these arguments may provide a way for theists to understand the nature of their god, they do not achieve their claimed goal of convincing all who give them due consideration. In that regard they are post hoc rather than evangelical.

Consistency of the notion of god

God is, on Anselm's account, something a greater than which cannot be conceived.

It would be worth taking some time to consider what goes along with one thing being greater than another. Two is greater than one, and infinity greater than every real number. So is infinity the greatest number? No, we have the transfinite numbers. Is there a number a greater than which cannot be conceived? For any number that one might conceive, it seems a larger can be found.

Surreal numbers contain all the reals, all the infinite and infinitesimal numbers; If we add imaginary numbers, do we have every number of which we can conceive? Are you brave, or foolish, enough to claim that there are no other numbers of which we might conceive?

My maths is not up to that task, but I want to make a more general point: those who place a limit on what can be conceived merely set up a target for others.

That's the trouble with Anselm's formulation. It lends itself to attempts at refutation. From the undergrad "can god make something so heavy that he can't lift it?" to theistic emanationism's committed to the existence of properties being posterior to God’s causing them to exist, Anselm created an argument, not a definition.

Further, it's just not at all apparent that the various attributes of god sit together in a coherent way; indeed, quite the opposite. The entire enterprise of theology appears for the most part to be an attempt to overcome this disunity, and with dubious - debatable - success.

Given these issues, it's not unreasonable to suppose that the notion of god is itself inconsistent.

Moral Considerations

Let's suppose that there is a god, and further that god's will is write in such a way as to be undeniable, as clear as day, so to speak.

Ought we do as he says?

Surely what god says is the case, is the case - it follows from his being who he is; and so it is argued that we ought do as he says. We do what is right by being obedient to his will, by submission.

Here's a philosophical question to consider: ought we do it because it is the will of god, or ought we do it because it is the right thing to do? We can put this in a slightly different way, by asking, is it is right to do as god wills?

Now this of course is Moore's open question, as applied to god. And to be sure it is open to the theist to simply say, yes, it is right to do gods will; and further that gods will and what is right are exactly the same thing. The theist may claim that the notion of what god wills, and the notion of what is the right thing to do, are the very same notion. But this looks to me like a conceptual error, since it is not on the face of it clear that what god wills and what is the right thing to do are the same notion; when we talk about what god wills, we are not, it seems to me, also talking about what it is right to do. That is, it seems to me that the question "Is it right to do as god wills" is not an analytic question; it is not like asking 'Is 2+2 the same as 4?" or "Are all bachelors unmarried men?". Abraham was willing to sacrifice his son as an act of submission or out of fear for god. Sacrificing another human being out of fear or obedience is not a virtuous act. Hence for the purposes of our story, it is possible to question the virtue of the will of god; and it follows that the will of god is not the very same as what is the right thing to do.

Now this is not a knock-down argument, since it remains open for the theist to insist that it was right for Abraham to sacrifice his son. But I don't agree. Further, I think those who think it was morally right for god to make such a demand are in error.

Even if a moral law were indisputably laid down by the good lord, it would remain open for people to choose to obey or not. The religious person still has to choose, and are not in a different position to the non-religious in that regard. So if the choice of a non-believer is in some way arbitrary, so is the choice of the believer. You cannot avoid responsibility for your moral choices by blaming god. So no, the choice is yours, regardless of there being or not being a god.

Bring to mind that all of this discussion presupposes that we have some direct access to the will of god. Of course, we have no such access, but are instead left to decide if this or that is the will of god, to choose between Jesus, Allah and Brahmin, and then to further choose between the minuscule variations therein.

All of which might count for nothing if it were clear that the actions of believers were more virtuous than those of unbelievers. But that is not what we see. To be sure, we do see people doing great and noble things in the name of their god, but we see others doing the same for love of their fellows. And we see dreadful crimes committed apparently under the auspices of the Good Lord. Ignoring the right of children, of women, of the dying while using institutional mechanisms to avoid responsibility. Believers do not have any special claim to virtue. They are all too much like everyone else.
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Some of the more pertinent replies:

What more really needs to be said ...
I contend that we are both atheists. I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours.
— Stephen Roberts?
In other words: Hitchens' Razor. — 180 Proof

From Salman Rushdie:

If you were an atheist, Birbal," the Emperor challenged his first minister, "what would you say to the true believers of all the great religions of the world?" Birbal was a devout Brahmin from Trivikrampur, but he answered unhesitatingly, "I would say to them that in my opinion they were all atheists as well; I merely believe in one god less than each of them." "How so?" the Emperor asked. "All true believers have good reasons for disbelieving in every god except their own," said Birbal. "And so it is they who, between them, give me all the reasons for believing in none.
The Enchantress of Florence — Fooloso4

I've always found it interesting that, even if the traditional "proofs" of the existence of God had any validity, they don't serve to prove the existence of a personal, peculiarly Christian God, although they're regularly touted by Christians and have been for centuries. The same would apply in the case of other personal Gods if the "proofs" are used to "prove" them. Even if they're true, there's still a long way to go to get from them to Jesus or any other personal deity. — Ciceronianus the White

In real life, it's the moral and political views that matter. I find it intensely uncomfortable that some of the most obnoxious and ignorant attitudes tend to go along with religious belief. Sorry, there's no philosophy here. I just wanted to say that. — bert1

In his book Five Proofs of the Existence of God, Catholic philosopher Edward Feser provides several long-winded metaphysical demonstrations of the existence of God:

the "Aristotelian proof" is 50 points long,
the "Neo-Platonic proof" is 38 points long,
the "Augustinian proof" is 29 points long.
the "Thomistic proof" is 36 points long.
the "Rationalist proof" is 27 points long. — darthbarracuda

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This was written in sections, then moved to the OP. Here are links to the original posts:
Arguments for the existence of god
Consistency of the notion of god
Moral considerations

Here are links to external, related threads:
Conceiving of agnosticism
Belief in god is necessary for being good