I had the thought today that the bible verse John 14:13 - "And whatsoever you shall ask in my name, that will I do, that the Father may be glorified in the Son"(Jesus) would be a great way to stump a Christian. Tell them that they should ask Jesus to convert you right then and there. Although I don't expect any sort of logical response, it begs the question why a believer can't get Jesus to perform a fundamental act of Christianity for someone who is trying to spread the "good news".