I've been arguing with a bunch of Baptists on the subject of (guess what) baptism. I discovered one question to ask to send them all running for the hills because they never expect it: where in scripture is the age of accountability? When they begin asking why is it important, point out to them that, if there is no age of accountability and one must make a decision to be saved, then God is a heartless bastard because no one under the age of, say, 7, just for the sake of arguing, is able to be saved. Considering the scriptural support for such a concept is nil, you have them. They will squirm and try to throw red herrings out. Keep on target and they will either stop talking to you (most likely) or they will relent and suddenly find themselves Lutheran. Use this advice wisely and have fun.