How do you say the word believer in Latin?

The term used in Christian Latin is credens, which is the present participle of the verb credere, "to believe". It means literally "believing [person]". In the plural (credentes) it's the equivalent of the English collective noun "the faithful".

There is also the Word acceptor, literally "one who receives" but used, at least in pre-Classical times (e.g., by the playwright Titus Macchius Plautus), to mean "one who accepts [something] as true". This Word does not have specifically religious connotations. However, it may be worth noting that acceptor suffered a change in Christian Latin, where it came to mean "one who favors [someone] unjustly".