Okay, I see a problem here, sleestak. You don’t want to talk to your friend about religion, but when he brings up the subject, you proceed to talk to your friend about religion!
By asking him for proof and engaging him in debate, you’re only feeding the fire. If you want him to stop talking about religion, then you’d better stop talking about it.
It’s obvious that you’re never going to sway him, because at this point, he’s impervious to reason. The more you try to tell him he’s wrong, the more he’ll feel compelled to keep talking about it to try to convince you he’s right. He doesn’t know how to win the debate by logic, so he’s going to try to win through sheer persistence. The only solution is to just disengage.
No matter what outrageous and easily-refuted statement he makes, you must not rise to the bait. I know, I know, it’s hard, because, darn it, if he brings it up, shouldn’t you have a chance to say your peace, too? But if you really want him to stop, you have to stop, too. Writing a letter about it, or telling him firmly that you will not discuss this subject with him is a good start, but then you have to follow through. If he brings up religion, change the subject. If he persists, tell him you can’t continue the conversation, and hang up. Either he’ll figure it out, or your friendship will come to an end–but if you keep letting him suck you in to religious discussions, you know your friendship won’t survive.
When you write the letter or have the talk with him, don’t make it about religion. Don’t tell him why you don’t believe or what you do believe or what you think about his religion or the historical accuracy of the Bible or Jesus as a person or anything. That would just be continuing the debate–and worse, he could take it as an underhanded attempt to get the last word in. You must make it about your friendship. The only thing you should ever say to him about religion is that you cannot talk about it any more.
I hope that your friend will come to a point where he can be more thoughtful about his religion. If he does, perhaps he will be able to accept your feelings on the subject, even if he does not agree with them. However, I don’t think he can be brought to that point by arguing with you. You need to give him time. You’ll have to help him by not succumbing to the urge to try to discuss these things with him–even to defend yourself–until he shows that he’s ready to have a mature, respectful conversation about it.
Good luck to both of you.