Anselm of Canterbury was an insanely devious motherfucker who chilled out in the 11th century. His proof of God was so badass that no one could figure out a good answer to it for hundreds of years.
Here's how he would get you with it:
Anselm: Oh, hey there, Mr. Atheist — quick question for you — do you accept that if there is a God that our understanding of Him is as a being who is so fucking awesome that it would be impossible to imagine something more awesome?
You: Sure, OK, whatever, but I still don't think that being actually exists.
Anselm: Yeah, but your idea of it exists in your mind, doesn't it?
You: Yeeeeeeah, so what?
Anselm: Well, I was just thinking that something that exists in reality is even better than some dumb thing that only exists in your mind. Right?
You: Duh, yes, obvs. Where are you going with this, Anselm?
Anselm: Where I am going with this is that if God exists in your mind, which you ALREADY ADMITTED THAT HE DOES, then we can imagine something that's even better than that — a God that exists in reality.
Anselm: Remember how we started off by agreeing that we can't imagine something better than God?
Anselm: Yeah. You just got owned. QED, bitch.