believer:
(plural: ‘believers‘)
a person who believes in something or someone, i.e. accepts something or someone as real or true and follows and supports them; a follower; a supporter;
a person who has great faith in something, especially in some religion; a worshipper,
If you are a believer in something or someone, you think that something or someone is good, right, useful. (from dictionary.reverso.net)
Once the entire Soviet Union was full of believers in communism, but eventually, some of those believers saw that there was something very wrong in their blind belief, and so they stopped believing in communism.
A religious believer is a person who believes, without giving much thought, reason or logic, that their religion and their god are the best and the rest of the religions and gods are the worst!

