I believe because of the witness of the martyrs, the apostles, and the successors of the apostles.
And what the hell does it mean to make your argument "Christian free?"
I thought it was funny. I disagree, but it’s still funny. But perhaps a live delivery would have gotten across the humor better.
BTW, when arguing with non-Christians on such subjects it doesn’t do any good to quote the Bible or other Christian source as an authority. The use of appeal to authority in a debate always a logical fallacy. However, it can have subjective weight in a discussion if both sides accept the source as an authority on the subject, but of course a non-Christian doesn’t accept the Bible as authoritative so it is useless.