The Idea of God

"The whole force of the argument rests on the fact that I recognize that it would be impossible for me to exist, being of such a nature as I am (namely, having in me the idea of God), unless God did in fact exist." Meditation 3, A&T 52

I'm not sure that I understand how Descartes can say that God's existence is based on him having the idea of God. In the first meditation, Descartes says,

"For indeed when painters themselves wish to represent sirens and satyrs by means of especially bizarre forms, they surely cannot assign to them utterly new natures. Rather, they simply fuse together the members of various animals. Or if perhaps they concoct something so utterly novel that nothing like it has ever been seen before (and thus is something utterly fictitious and false), yet certainly at the very least the colors from which they fashion it ought to be true." etc. Meditation 1, A&T 20

So basically I do not understand how the idea of God has to mean that there is a God. Why couldn't the idea of God come from a combination of ideas, or be something "utterly novel?" Maybe I missed a big part of his explanation, but this is what stood out to me when I finished.