He never misses a chance to defend Islam, but he is silent about Jesus and the martyrdom of Christians at the hands of Muslims. That’s why few think he’s a Christian.
Too bad not enough of any who might care (which admittedly could be none of them) didn’t realize it sooner than that, like when he said to George Stephanopoulos in his first White House campaign “my Muslim Faith”, and Georgie-boy helpfully corrected him to recall that he meant “my Christian faith”.
Recall during the '08 campaign he was asked "Who was Jesus Christ?".
His initial answer was "A great teacher". Not, "Son of God, sent to earth to atone for the sins of mankind" or something like that. He did say something about "Son of God" at the very end of his answer, as an afterthought, but it was just an afterthought.
He's clearly not a Christian.
I don't think he's Muslim, either, because being Muslim requires worship of Allah, and The Won worships none but himself.