Dumber than dogcr@p, racist, religious fanatic Bob Jones III (BJ3) has come out from under his rock yet again with the laughably stupid rhetoric of questioning, "Where is the evidence that [Obama] is a Christian?". Interestingly, Glenn Beck and Rush Limbaugh have both made similar charges one or more years ago and these charges have been thoroughly refuted. So apparently BJ3 can't be bothered to do any research there either. The answer is that the evidence is plentiful, in Obama's case. Here is the Lincoln Bible used by Obama in his Presidential Inauguration.
Then there was his attendance at Trinity United Church of Christ on 95th street in Chicago, which was used against him during the last campaign, because of a certain Reverend at that church named Jeremiah Wright. After his falling out with Wright, which was due to politics, not religion, he has been documented attending churches throughout Washington, DC, including Allen Chapel, and St. John's. He has held prayer vigils, prayer breakfasts and given numerous speeches where he has affirmed tenets of Christianity. The de rigeur right-wing claims that Obama is a Muslim have been thoroughly debunked by snopes.com, as well as articles in politiFact.com, and others. Even the claim that Obama was "once a muslim" or that he "professed to be a muslim" have been thoroughly debunked. The claim that Obama was converted to Islam by his father is belied by the fact that his father left when Obama was 2 years old, and his father was a "confirmed atheist", not a Muslim, by the time his parents met.
In speeches, like this one, at the 58th annual National Prayer Breakfast in February of 2011 Obama described his beliefs in detail, talking about daily praying for humility, guidance, and wisdom with his wife who is also unquestionably Christian. So the answer is that the evidence is easily available to anyone who cares to look, but many people like BJ3 have dogmatic opinions that no amount of evidence will sway.