believer
n.someone who is sure that something is true or good. You can use this for religious faith or just for having strong confidence in an idea.
n.a person who maintains a firm conviction in the truth or efficacy of something. Often followed by the preposition 'in' to specify the object of faith or confidence.
“She is a firm believer in the power of daily exercise.”
“As a true believer in lifelong learning, he enrolled in a new university course at the age of eighty.”
“While some critics remained skeptical of the new economic policy, the prime minister found a staunch believer in the head of the central bank.”
Commonly pairs with the preposition 'in' to indicate the subject of the belief.
The noun believer almost always takes the preposition 'in' rather than 'of' when describing the object of belief.
From Middle English bilevar, bileever, belever, equivalent to believe + -er.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.