I think quite a lot of it. No doubt, he wouldn't have the opportunities he has had at the end of games if his defense wasn't holding opponents to just a few scores, and if his kicker wasn't making amazing kicks. But it's very clear that he plays his best when the game is on the line, and that he has led a whole series of spectacular, game-winning drives. That's no fluke.
Beyond that, he just has "it," in the same way that a guy like Tom Brady does. He wins, and believes he is going to win. This is something that spreads through the whole team when someone at the QB position has it - the defense plays better, the kicker starts to believe, the fans start to believe. It's a real intangible.
Dont forget he prays in a very visable way.