In a nutshell Descartes' "proof" is derived from a definition of God! Simply, God must exist by definition. But that assumes his existence. If it were true, then "God" is the only thing proven to exist by definition.
For that matter, if the entity (God, in this case) is already KNOWN to exist the 'argument' is superfluous as well as fallacious. What if I imagined or conceived of a 'Big Blue Meanie' and attributed to it the property of existence? I could simply assert that 'big blue meanies' exist by my definition. Someone similarly conceived and defined 'God'. It was left to Descartes to provide the process with an imprimatur of legitimacy and 'logic'.
Descartes often compares the ontological argument to a geometric demonstration, arguing that necessary existence cannot be excluded from idea of God anymore than the fact that its angles equal two right angles, for example, can be excluded from the idea of a triangle. The analogy underscores once again the argument's supreme simplicity. God's existence is purported to be as obvious and self-evident as the most basic mathematical truth. It also attempts to show how the “logic” of the demonstration is rooted in our ordinary reasoning practices.
--Stanford University Encyclopedia of PhilosophyDescartes believed the ontological argument to be a "proof" that followed from the "essence" of "God"! But that assumes the existence of 'God'; or --as lawyers would put it to a judge --it assumes 'facts not in evidence'. If God's existence is in doubt, if it is in "need" of proof, then how may we know that "God" possesses an essence?
It's a circular argument: God exists because God exists! There is much more that can be written about this but, alas, it will not dissuade those who want to believe and who will tolerate fallacies in order to do so. I write this short essay not because I am an 'atheist'. "Atheism" is as irrational as is religion. One cannot prove that God does not exist because nothing meaningful can be said of anything which does not exist. For example: "Blue Meanies do not exist" is meaningless.