Whenever someone asks me if I am Christian (or claims that I'm not), I try to say something like:
"That term ('Christian') means different things to different people. I believe I am a Christian, but I'm not sure if you would or not. Do you have a few minutes to listen to me explain my beliefs and see if you think I am?"
If they are serious and really want to know, they will listen - or pick a different time to do so. If they aren't interested in making that decision for themselves (or have made it already), they will turn me down - and that will be it. No debates, no hard feelings, no arguments, no change from before the question was asked. Nothing gained, but nothing lost.
That's what I try to do when someone questions my Christianity.