A good reason that, for example, the Catholic God exists
? I'm don't think there is one. But that's just a rejection of the idea that the concept of God is such that it can be meaningfully attributed "existence". Kierkegaard said that "To stand on one leg and prove God's existence is a very different thing from going down on one's knees and thanking him.
" I believe that most Catholics do the latter, and those trying to do the former probably have read too much philosophy.
From what I understand, Paul Tillich is a relatively famous theologian who rejects God's "existence" also. I haven't read his works myself, so I can't comment much more than that yet. I'm sure I'll get around to him eventually.