Having concluded that God must necessarily exist, the Meditator asks how he received the idea of God. The idea cannot be adventitious, coming from without, nor can it be invented by the Meditator. Thus, the idea must be innate, and the Meditator must have been created by God with this idea already in him. He clearly and distinctly perceives that God is no deceiver, since all deception relies on some defect or other, and a perfect God has no defects.
Now that we have reached the end of the Third Meditation, we can more easily review the overall strategy that Descartes is pursuing. The Meditation begins with the Meditator certain only that he exists and that he is a thinking thing. He concludes that he comes to know these facts through clear and distinct perception, and reasons that it should follow that all his other clear and distinct perceptions are true. In order to confirm the truth of clear and distinct perceptions, however, he must prove the existence of a benevolent God. If God were a deceiver, he could be deceived even with respect to his clear and distinct perceptions.
However, the proof of the existence of God relies on the Meditator's having a clear and distinct perception of the idea of God. The proof seems to fall into what is now called the "Cartesian Circle." The Meditator seems committed to claiming both (a) that we can only be sure of our clear and distinct perceptions if God exists and (b) we can know that God exists because we clearly and distinctly perceive the idea of God. If both (a) and (b) are true, Descartes is guilty of circular reasoning.
There are number of ways in which we could attempt to release Descartes from this circle. One strategy, called the "Cartesian spiral," is to suggest that the clear and distinct perceptions going into the proof of God's existence are different from the ones that follow from it. For instance, my clear and distinct perception that 2 + 3 = 5 can be doubted unless God confirms it, but my clear and distinct perception of the idea of God is somehow immune from doubt. In this reading, there are different kinds of clear and distinct perceptions, some of which are totally immune from doubt and some of which need God to confirm them. This reading is made plausible by the fact that my clear and distinct perception that 2 + 3 = 5 is a judgment and therefore open to error, whereas my clear and distinct perception that God exists is simply an idea in the strict sense, with no judgment attached.
Another strategy is to re-evaluate the epistemological role that God is meant to play in the Meditations. According to this reading, God cannot possibly be intended by Descartes as confirmation of clear and distinct perceptions. If that were the case, it would be a lost cause to try to prove God's existence by means of the intellect, since we would not be able to prove anything by means of the intellect until we know that God exists. Rather than seeing God as the confirmation of clear and distinct perceptions, we could read God as a buffer against doubt. We know clear and distinct perceptions independently of God, but God's existence also provides us with the certainty that we might not otherwise have. In this reading, (b) is true, but we would re-formulate (a) as saying that we can re-affirm our clear and distinct perceptions retrospectively once we are certain that God exists. The problem with this reading is that it totally re-structures the way we understand the Meditations: clear and distinct perceptions, and not God, become the ultimate foundation for knowledge.
We should note that, in spite of the revolutionary originality of much of the Meditations, Descartes' proofs of the existence of God are derivative of proofs that were popular among the Scholastic philosophers. This proof relies on causal reasoning, suggesting that there must be a cause of the idea of God that is as great as God himself. Though my idea of God might have come from my father, and my father's idea of God might have come from a priest, the suggestion is that at the end of that causal chain, there is a first cause, which is God. The proof of the first cause is usually used in pointing out that there must be some unmoved mover at the source of all change in this universe. However, this proof has since been discredited, since it relies on a faulty understanding of causation which assumes, among other things, that all causal chains must have a first term.