Harry Potter

I'm not from the US, so that does not apply.

It became popular here (talking about the first book before the others were written) because it was a well written story.

It got a lot of people, both adults and kids, who did not normally read for pleasure to read it. I don't remember hearing much about the religious stuff in the USA but I think part of its success was that people who didn't read much read it and liked it.