Originally, the term 'bible thumpers' was used to describe people who tapped on their Bibles during a preacher's sermon
to remind him to stick to the Bible
, and not go off on his own little tangent
It has since become a derogatory term used against Christians, profiling them as brainless "ditto" heads who could not explain their own beliefs, but agree with everything the preacher says. Oh, and there are some people like that. But just because someone stupid believes in something, doesn't mean that that thing has no merit.