Descartes in meditation three states his reasoning for the existence of God. The concept he uses to justify the existence of God is that God is so unique that to have the ability to think of a being of infinite power and wisdom must mean that that being exists. The idea is that something of less complexity, our minds or are being in general, can't produce the idea of something more complex, God or a God like being. This concept doesn't coincide with my inability to justify the existence of God through logic.
To understand Descartes justification for the existence of God one must first have some general idea of Descartes' philosophy. In particular first we should examine the way in which Descartes describes or classifies thoughts and other mental processes. Descartes associates "ideas" with our concept of objects or images. Our concept of things like trees, houses, books, etc.
These things are familiar to us and have a definite form that can be recalled. Descartes also has another classification of thoughts for those that go beyond the simple, familiar, concept that we have of them.
These are other thoughts that can't be classified as simple "ideas". These are things like fear, or love. This classification of thoughts also includes things like denial or affirmation. According to Descartes these thoughts are classified as "affections" or "judgements". There are many things that Descartes feels can be attributed to these different classifications of thought with absolute certainty.
The first, and most substantial, attribute that can be assigned to "ideas" is the simple fact that they cannot be false. "They cannot, properly speaking, be false; for, whether I imagine a goat or chimera, it is not less true that I imagine the one or the other" clearly states how Descartes denies...