Rene Descartes proves the existence of God using his argument of the “Cogito Ergo Sum” as the starting point.  He claims that God exists because it is God Himself who implanted the idea God to his (Descartes) mind. 

When Descartes thinks the idea of God, he proves his “thinking” by examining his own thinking of himself.  Thus, he says, Cogito ergo sum, I think, therefore, I am. 

In connection with this claim, when he grasped the idea of God who is greater than himself, the idea of God which is present in Descartes’ mind comes from God Himself.  He (Descartes) said this simply because, he cannot think of a greater idea of God since he (Descartes) is less great than his own idea of God. 

So, where does this idea of God come from?  Logically, it comes from God Himself simply because (not that simple for some people); God Himself is the Cause of the Effect of the idea of God on him (Descartes). 

Thus, there would be no greater effect than the cause.  Since, God is the cause of the idea of God, it follows that God exists.  Descartes cannot think of the idea of God, if his idea of God comes solely from him. 

In a nutshell, Descartes says that God exists because he has in his mind the idea of God, and this idea of God cannot come from him since this idea (of God) is greater than him.  If that is the case, there is innate idea of God in his mind implanted by God Himself. 

Therefore, God exists.