Theories of Knowledge

I found that Meditation 3 was a little more difficult for me to digest than the previous. It could have been that it was longer, but also because I myself do not believe in God.
Descartes says that if there were not a God, and we created ourselves that we would have made ourselves perfect. But, if we did make ourselves the way we are, and had no concept of God existing than how would we have known what was perfect in the first place? To me, saying that there has to be a God, otherwise we would have made ourselves "perfect" doesn't make sense because we would not have had anything to judge what was perfect or not.Maybe this is what we did think was perfect.....
Also, because we are capable of the thought of God, does this mean that unicorns are leprecons are really in existence? Because we have the thought, did they once really exist, we just have to trace the ideas back far enough?