Amazing why this question continually comes up.
I can only think of one reason:
1.) Because his name is Barack Hussein Obama
If his hame was Barry Osteen, it wouldn't be an issue. It's amazing how so many people believe
that you're not a Christian unless your name is James, Peter, John, or Matthew - as if these were
the actual names of these people in the bible to begin with. Christianity in the west unfortunately
has become Anglicanized, and some simply can't see someone with a name like his being a Christian,
and being president.
Few presidents in the past - outside of JFK had to "defend" their faith to the level Barack, and for that
matter, Mitt Romney have. Whether these former presidents were "devout" or practicing really didn't matter.
Most of these presidents have issues in their private lives that would question whether their faith was genuine - either in their executive decisions, or affiars, or whatever.
And in Republican camps, as long you answer a few questions in the affirmative - e.g. abortion, guns, homosexuality, etc., that's all their faith seems to boil down to.
John McCain has done some pretty "un-Christian" things in his life, which he has never come full mea-culpa on. I found it interesting in the interview with Rick Warren, when asked what his greatest moral failing was, he said the failure of his first marriage. Note he did not say that he cheated on his wife multiple times, and started seeing his current wife while still married to his first wife.
If people want to question whether Barack is a true Christian based on his words, values, and voting record, perhaps that's fair, but then again, if you go by the Christian requirements of faith - that you confess with your mouth and believe in your heart that Jesus is Lord, and that Jesus died and was buried, and was born again, that you will be saved - i.e., you become a Christian (Romans 10:9 - 10), who can truly judge then? He's obviously confessed with his mouth. Who has access to his heart?