The proof for the existence of God that is the most believable in my opinion is Descartes' Ontological Argument. He states that God's existence is contingent directly from the fact that necessary existence is restricted in the clear and distinct idea of a supremely perfect being. He explains it best in his Fifth Meditation where he says:
"But if the mere fact that I can produce from my thought the idea of something entails that everything which I clearly and distinctly perceive to belong to that thing really does belong to it, is not this a possible basis for another argument to prove the existence of God? Certainly, the idea of God, or a supremely perfect being, is one that I find within me just as surely as the idea of any shape or number. And my understanding that it belongs to his nature that he always exists is no less clear and distinct than is the case when I prove of any shape or number that some property belongs to its nature" (Rene Descartes)
Rene Descartes relates his philosophy for existence of god in the same way he regards a triangle.
The existence of God doesn't require proof. Just as there is no proof that triangles are real or that anything in the world is triangular. Descartes said "I found that this included the existence of such a being, in the same way as the idea of a triangle includes the equidistance of all parts (of its surface) from the center; or indeed in an even more evident way" Anthony Kenney Western Philosophy
According to the Stanford encyclopedia of philosophy "Descartes also characterizes the ontological argument as a proof from the "essence" or "nature" of God, arguing that necessary existence cannot be separated from the essence...