Proof of God-The Ontological Argument

These are straight from the source...

1. St. Anselm (around 1077) developed what is now called the "Ontological Argument" for the Existence of God in his book "Faith Seeking Understanding." He argued:
Men believe God to be the Being than which none greater can be thought. It is greater to exist in reality and in the understanding than to exist in the understanding alone. Therefore, it is contradictory to hold that God exists only in the intellect, for then the being than which none greater can be thought is one than which a greater can be thought, namely, one that exists both in reality and in the understanding. 2. Descartes:"Whatever is contained in a clear and distinct idea of a thing must be predicated of that thing; but a clear and distinct idea of an absolutely perfect Being contains the notion of actual existence; therefore since we have the idea of an absolutely perfect Being such a Being must really exist."

3. Leibniz:"God is at least possible since the concept of Him as the Infinite implies no contradiction; but if He is possible He must exist because the concept of Him involves existence."

Thanks to: http://www.self-realization.com/articles/proof_God.htm