You aren't getting what we're saying. It's not about what religion the president is as long as he doesn't let his religious beliefs decide how the government is run or push them on the country. So Kennedy was right when he talked about separation of church and state. People were afraid back then that the Pope or the Catholic church would be controlling him or telling him what to do and he said no, that wouldn't happen.
I think Santorum is just saying this stuff because he's trying to play to the fundamentalist evangelicals. He's twisting around the meaning of separation of church and state to make himself look like a defender of Christianity. It must have worked in Miss and Alabama but it could be a tactical error on his part in the long run because Evangelicals don't like Catholics much as you can see by comments here on glp that are made all the time. They don't want a Catholic who they think might take orders from Rome. So by denying that religion and politics should be kept separate he may be getting some people upset.