What Descartes is essentially doing in meditation 5 is going about what is clearly, distinctively true and what is not. He believes anything that leaves you in the least bit doubtful must not be 100% true, whereas early he would act decisive even if his thoughts were essentially uncertain. At one point he questioned his own existence, and came to the conclusion that doubt requires thought, and thorugh requires existence, therefore he is thinking, which means he exists. Althought I do not agree with his theory on God's existence. He doesn't really explain why he thinks God exists. He just claims that a triangle has 3 angles that equals 180 degrees, and that God's existence is as much an essential property of God's as having three angles that add up to 180 degrees is an essential property of triangles. That makes no sense.