does tom really change when he becomes religious?

No, he doesn't. He only does this to weasel out of his deal with the devil. When Old Scratch approaches Walker to collect on his own promise, Walker realizes that he must pay up and be responsible for his own promissory note. Only then does Walker become pious and churchgoing to prove to the Devil that he has seen the light. Unfortunately, his religious conversion has not helped him one bit because he is critical of everyone in the church, quick to judge them, and refuses to see the error of his ways. But Walker has achieved his wealth through greed, and as a result he becomes a prisoner of his own doing.