We can object that there is a gap between the idea that God exists eternally and God actually existing eternally. Descartes is aware of this, and objects to himself:
just as it does not follow that there is any mountain in the world merely because I conceive a mountain with a valley, so likewise, though I conceive God as existing, it does not seem to follow on that account that God exists.
But, he replies,
the cases are not analogous… it does not follow that there is any mountain or valley in existence, but simply that the mountain or valley, whether they do or do not exist, are inseparable from each other; whereas, on the other hand, because I cannot conceive God unless as existing, it follows that existence is inseparable from him, and therefore that he really exists.
Descartes is arguing that the analogy is not between mountains and existence and God and existence; but between mountains and valleys and God and existence. The idea of existence is no part of the idea of a mountain. But just as the idea of a valley is implied by the idea of a mountain, so the idea of existence is part of the idea of God. And so, as he says, I can’t think of God without thinking that God exists.
But what does this show? Just because I can’t think of God not existing, does that have any relevance to whether or not God exists? Absolutely. The bounds of our thought are, at least on some occasions, indications of what is possible. This isn’t because our thought creates or influences reality, but because thought reveals reality. And so, Descartes argues, the necessary connection between God and existence isn’t something I’ve come up with, it is something I discover:
the necessity which lies in the thing itself, that is, the necessity of the existence of God, determines me to think in this way: for it is not in my power to conceive a God without existence.
There is a conceptual connection between the concept of God and God’s existence, and this entails that God’s must exist.