This question is probably on the verge of being blasphemous, but I'll just assume Mike from Mississauga saw Our Lord and Saviour wearing a jersey with "H. Christ" on the back and was legitimately curious.
I can think of some things it probably doesn't stand for:
• Hayden - Too trendy
• Hortense - Too untrendy (and feminine)
• Hamburglar - Too cool
• Homer - D'oh!
• Hank - I dunno, just seems unlikely
• Hussein - Come on, no one could become so popular with this middle name!
A safe answer might be "Holy" I seem to recall songs saying "Holy is your name" but that's just speculation.