believer

Concise Oxford English Dictionary © 2008 Oxford University Press:
believe/bɪˈliːv/

▶verb

• 1 feel sure that (something) is true.

  ■ accept the statement of (someone) as true.

  ■ have religious faith.

• 2 (believe in) have faith in the truth or existence of.

• 3 (believe in) have confidence in.

• 4 think or suppose: I believe we've already met.

  ■ (believe in) think that (something) is right or acceptable.

– derivatives
believability noun,
believable adjective,
believably adverb,
believer noun.

– origin OE belȳfan, belēfan, alt. of gelēfan, of Gmc origin.