In his philosophical system, Leibniz tried to juggle God and the monads. He claimed that monads are both eternal & indestructible, yet also with the same breath, he claimed that God is capable of creating them or destroying them in a jiffy. Monads are supposedly free, which is a must for all substances, but not so in God’s eyes.
Now, just exactly what is God in this system of monadology? Is God a monad Himself? Leibniz only goes far enough to assert that God is the “monad of monads.” Interesting, isn’t it?
If God isn’t a monad, then that would explain for God’s existence before he “flashes” the monads into existence. However, that means monads exist, and consists of properties only in virtue of the properties of this flashing, non-monadic entity. Then if monads depends on another entity, then they are not substances. The definition of substance is something that does not depend on anything else to be what it is. If monads depends on God, then they are only “modes” of a substance. If God is the only entity that does not depend on anything else to be what it is, then God is the only substance.
Hegel observed this: “There is a contradiction present. If the monad of monads, God, is the absolute substance, and individual monads are created through his will, their substantiality comes to an end.” (Lectures on the History of Philosophy, 3rd section)
If God is not a monad, then Leibniz is basically a Spinozist.
On the other hand, let’s say the “monad of monads” is a monad. However, if God is a monad, then by definition he cannot interact with other monads, or he would determine their essence, and/or they, his. At best, God as monad may be involved with his creation only virtually, which is the pre-established harmony Leibniz often writes about. Thus, if God acts through this pre-established harmony, then he could not have created that either. FWIW, whatever God does, it follows with absolute, logical necessity from his monadic essence. The idea that God will “create” this universe is already contained within the concept of God, much like “got shot in Dallas” is a necessary predicate of “JFK.” Given the hypothesis that to choose one monad is to choose the entire universe, then therefore, once God exists, then the universe such as it is, exists with utter necessity. God could not have had a choice about anything – except like JFK, he is ignorant of his true nature.
Bottom line: if God is a monad, then he isn’t God, but someone like us. If God is a monad, then Leibniz was an atheist.