And that proof is … himself. No, seriously. It's in his latest book
Next time you meet an atheist, tell him or her that you know a bold, fresh guy, a barbarian who was raised in a working-class home and retains the lessons he learned there. Then mention to that atheist that this guy is now watched and listened to, on a daily basis, by millions of people all over the world and, to boot, sells millions of books. Then, while the non-believer is digesting all that, ask him or her if they still don't believe there's a God!
Bill, Bill, Bill … the fact that you sell millions of books is not proof that God exists; it's proof that P.T. Barnum was an optimist