I think the mother in Carrie was always heavy into religion all her life... along the strains of the Puritans. Repent ye sinners! My little understanding of the Puritans' faith was that it was along the lines that any enjoyment of life is most likely a sin and must be avoided. Lust is the first of the seven deadly sins. All you should use your mind for is praise of God. That sort of thing.
I've never felt that King had a general animosity toward any religion, including Christianity, but I think he was critical of religion when it would be used as a tool to drive a woman clinically insane with guilt and lead to her ruining her daughter's life.
King is a story-teller. I'm guessing he thought that readers could relate to hearing of people, or even having acquaintences, that seem to be just a leetle too into their religion.
It may be a bit more than that. Here’s some quotes I found:
“I’m not sure there is an afterlife. OK. If there is one, here’s what I think it is. I think it’s whatever you think you’re going to get. Those suicide bombers, if they really believe that they are going to wind up in heaven with 71 virgins, yeah, that’s probably what they’re going to get in the afterlife.”
“The other thing that’s interested me ever since I was a kid was the idea that’s baldly articulated in “Desperation,” and that is that God is cruel. I always in my mind equated Mother Abagail with Moses, and the story of Moses taking credit for the water coming from the rock and being forbidden to get to the Promised Land because of that one thing, that one slip, where God is cruel, and I wanted to use those things and say two things. First, that the myths are difficult and suggest a difficult moral path through life, and second, that they are ultimately more fruitful and more earth-friendly than the god of technology, the god of the microchip, the god of the cellphone.”
“I’m interested in the concepts. I’m particularly interested in the idea that in the New Testament, you’re suggesting a moral code that’s actually enlightened. Basically what Christ preached: get along with your neighbor and give everything away and follow me. So we’re talking pretty much about communism or socialism, all the things that the good Christian Republicans in the House of Representatives today are railing about in light of this bailout bill. Of course, Christ never preached give away everything to Wall Street, so they might have a point.”
Sounds like he has issues. God is cruel, suicide bombers will get their virgins, and Jesus preached communism. What a witches’ brew.
http://www.salon.com/books/int/2008/10/23/stephen_king