true believer

plural true believers

1. A strict follower of a doctrine.

   The sudden and stunning transformation of a MAGA true believer also landed Ms. [Marjorie Taylor] Greene in places she would never before have been invited, and earned her strange, new respect from media outlets and lawmakers who had never before wanted to associate with her.

   2025 November 22, Annie Karni, “Marjorie Taylor Greene Says She Will Resign in January, After Break From Trump”, in The New York Times, New York, N.Y.: The New York Times Company, →ISSN, →OCLC

   1 more example

   When he had repoſed four years and four months in the boſom of care, rocked in the cradle of proſperity, he was brought forth, according to the cuſtom of true believers, and committed to the charge of moralifts of high fame, poliſhers of noble manners, and ſearchers of true knowledge, that they might cultivate his heaven-born genius, and teach him with dignity to rule that world which he was born to command[.]

   1769, Firishta, translated by Alexander Dow, Tales translated from the Persian of Inatulla of Delhi, volume I, Dublin: P. and W. Wilson et al., page 4

2. An individual who maintains unwavering belief in a claim irrespective of empirical evidence, including clear or conclusive proof of its falsity or fabrication.
3. Used other than figuratively or idiomatically: see true, believer.