'believer' originates from Old English, specifically the verb 'belīfan' (or forms like 'belēfan'), where the prefix 'be-' was intensive and 'līfan' meant 'to trust; to accept as true'.

'believer' changed through Middle English (forms such as 'beleven', 'bileve') with the agentive suffix '-er' added to the verb 'believe', producing the noun 'believer' meaning 'one who believes'.

Initially it meant 'one who accepts something as true'; over time it has taken on the common sense of 'a person who holds a religious faith' or more generally 'a person who strongly supports a particular idea'.