(religion) Any of those who separated from the official Russian Orthodox Church after 1666 as a protest against church reforms introduced by Patriarch Nikon between 1652 and 1666. They continue liturgical practices which predate the reforms.

plural of make believe

(rare) Someone who hides their belief.

plural of crypto-believer

Be assured; be certain. / Used other than figuratively or idiomatically: see believe, me.

someone who shares one's faith; a brother in faith

(idiomatic) An emphatic form of believe me: you (the subject) had better believe me (the speaker).

Alternative spelling of true-believer syndrome

A phenomenon in which witnesses to a staged event continue to believe in its authenticity even after it has been debunked.

(idiomatic, chiefly in the negative) To believe that something which one directly sees is truly the case.