A believer or adherent — someone who believes in and follows a religion, ideology, or sect.
彼はその宗教の熱心な信者だ。
He is a devout believer of that religion.
信者の数は年々増えているという報告があった。
There was a report that the number of adherents has been increasing year by year.
教会には多くの信者が集まった。
Many believers gathered at the church.
Colloquial: a fanatic or superfan who is extremely devoted to something or someone (e.g., 'Apple 信者' for an Apple fan).
彼はそのバンドの信者で、コンサートには毎回行く。
He's a superfan of that band and attends every concert.
ネット上には特定メーカーの『信者』が多い。
There are many 'fanboys' of certain manufacturers online.