If you had a Muslim childhood, have been a member of a Christian church for 20 years and somebody calls you a Muslim, then you feel offended, you might not really be a Christian but a Muslim pretending to blend in with your enemies.
I would suggest that a real Christian with Obama's apparent oratory skills should welcome an opportunity to explain what Christianity means to him. Not trying to play "holier than thou", since Christianity is rightly humbling, but rather just offering some simple personal insights.
While having to interview dozens of people so as to prove to each of them that one is a Christian would get rather annoying, I would think that a public speech or two wouldn't be unreasonable.
Besides, I would suggest that it would in general be a good thing for politicians to acknowledge how they would expect their religious views to affect--or not affect--their policies. That would seem to be relevant information for a voter picking a candidate.