Descartes makes it obvious in the very beginning of his third mediation on page 503 that what he knows is clearly true. Though he has doubted other things before, this time he is certain. Certainty comes from senses, reason and faith. Descartes states that if he is not certain about God, then he can not be certain about anything else (pg 504). He says that ideas of God are treated the same as any other term and is not necessarily true or false. It is strictly an image. Even if the image is non-existent, it does not make it any less desirable or less true.
Can one really prove there is God? Proving something or someone does not exist is harder than proving something does exist. It becomes easier to say that there is a God, than to say that there isn't. For example, not seeing, feeling, hearing, smelling or tasting something, does not mean that it can not be seen, felt, heard, smelt, or tasted. Just because you haven't ever seen a million dollars, does not mean there is no such thing as a million dollars. The same can apply for God. He is believed to have taught us, yet do we really have proof. .
Some of Descartes ideas are innate. Innate ideas are ideas you were born knowing. They are ideas you don't have to develop; they are instilled in you at birth. Descartes recognizes that his ideas are not all dependent on his will, but are impulses from the outside of him (external things). He later discusses perfections and imperfections. His ideas have a lack of perfection because of deficiency. The deficiency is whether or not Descartes could distinguish something true, from a non-thing, such as God. This leads to separation of categories and substance. To Descartes a substance like God could exist in various classifications. The substances Descartes imagines, have only originated through him, and are merely substances he created. So could Descartes created have created God in his own mind.