People's religion is what they state it to be. Obama says he is a Christian. Therefore at this time he is a Christian.
From what I have read: Up until Obama was 31 he was a devout muslim. At that time to pacify his wife Michelle he professed to believe in Jesus Christ and joined Bro. Wright's Christian Church.
So, unless Obama decides to change his religion in the future, he is a Christian.
So, unless he says otherwise, get off of his case. For all practical purposes the man is a Christian.