This is quite a generalization. I am far from religious and loved Tebow as a college player. I don't think he can play QB in the NFL, but it has nothing to do with his beliefs.
So you also saw all of Orton's games. How is it that both of them had the same issues? Inability to throw over the middle, inaccurate and unable to score TDs in red zone. They are two completely different type of player.
What exactly is the risk here? If Harvin realizes that this is his last shot and plays the way he is capable of its a big win for the Bills. If not cut him. It's a one year deal.