This is partially in response to @mitchell saying Plato is not a monist but also sharing a solution to a key problem I've had in my metaphysics.
So there are different definitions of monism. Good old Wikipedia:

"Monism is the view that attributes oneness or singleness (Greek: μόνος) to a concept (e.g., existence). Various kinds of monism can be distinguished:

Priority monism states that all existing things go back to a source that is distinct from them (e.g., in Neoplatonism everything is derived from The One).[1] In this view only one thing is ontologically basic or prior to everything else.
Existence monism posits that, strictly speaking, there exists only a single thing (e.g., the universe), which can only be artificially and arbitrarily divided into many things.[2]
Substance monism ("stuff monism") asserts that a variety of existing things can be explained in terms of a single reality or substance.[3] Substance monism posits that only one kind of stuff (e.g., matter or mind) exists, although many things may be made out of this stuff."

Plato is clearly a priority monist, but not a substance monist (mind/matter Dualism).

But let's talk about existance monism. Reality doesn't appear at least at first glance to be all of one substance like water or something. Granted I don't understand quantum physics, but at least on a molecular level not same substance. Now Plotinus says everything is an emanation from the One and returns to the One. Spinoza says simmilarly that everything is a mode of God. Granting these ideas to be true it is then unclear if this is really a Pantheism or an Panentheism, ie is everything from and thru God or is everything God.

On the surface appearance, the universe and particularly humans do not seem very God like. I am a jerk, God is not a jerk, therefore I am not God. Yet what about on deeper level? What about my purified and detached soul that through mystical experience has the potential to be granted union with God? At that point I become completely enraputured by God and have no other thought but of Him. So far, I experience this mystically only partially but faith holds the promise that one day in death or in another realm or after many more lives I will attain this pefected, enlightened state. I will be so illumined with the light of God that my darkness will be completly gone.

Does this mean that in this state my inner divinity is reunited or actualized or something? Or does it only mean that I'm a creature who will one assume an elevated but non-divine station? The religions are split on the issue. But really there's no way to know. Even if I was able to actualize my true divinity, the I that experienced it wouldn't be divine. My human pysche is by definition human and will always experience being outside the Godhead, regardless how mystical I might become.
At the same time the goal and path of these two models is the exact same. Both are union with God and both require purification (thru God) in order to achieve such closeness to Divine Purity.

"Therefore we must ascend again towards the Good, the desired of every Soul. Anyone that has seen This, knows what I intend when I say that it is beautiful. Even the desire of it is to be desired as a Good. To attain it is for those that will take the upward path, who will set all their forces towards it, who will divest themselves of all that we have put on in our descent:- so, to those that approach the Holy Celebrations of the Mysteries, there are appointed purifications and the laying aside of the garments worn before, and the entry in nakedness- until, passing, on the upward way, all that is other than the God, each in the solitude of himself shall behold that solitary-dwelling Existence, the Apart, the Unmingled, the Pure, that from Which all things depend, for Which all look and live and act and know, the Source of Life and of Intellection and of Being." Plotinus the first Ennead