Believer

to have confidence in the truth, the existence, or the reliability of something, although without absolute proof that one is right in doing so:
Only if one believes in something can one act purposefully.
to have confidence or faith in the truth of (a positive assertion, story, etc.); give credence to.
to have confidence in the assertions of (a person).
to have a conviction that (a person or thing) is, has been, or will be engaged in a given action or involved in a given situation:
The fugitive is believed to be headed for the Mexican border.
to suppose or assume; understand (usually followed by a noun clause):
I believe that he has left town.
believe in,

to be persuaded of the truth or existence of:
to believe in Zoroastrianism; to believe in ghosts.
to have faith in the reliability, honesty, benevolence, etc., of:
I can help only if you believe in me.

make believe. make1 (def 68).
Contemporary Examples

The Book-Club Hustlers Francesca Mari July 5, 2009
The Jihadis’ American Pipeline Patrick Symmes October 2, 2011
Your Guide to the End of the World David A. Graham May 19, 2011
William H. Gass: How I Write Noah Charney March 12, 2013
Atheist Philosopher Peter Boghossian’s Guide to Converting Believers Michael Schulson November 1, 2013

Historical Examples

Where the Trail Divides Will Lillibridge
Samantha Among the Brethren, Part 4. Josiah Allen’s Wife (Marietta Holley)
Notes on the book of Exodus C. H. (Charles Henry) Mackintosh
The Emigrant Frederick William Thomas
A Contribution to The Critique Of The Political Economy Karl Marx

verb
(transitive; may take a clause as object) to accept (a statement, supposition, or opinion) as true: I believe God exists
(transitive) to accept the statement or opinion of (a person) as true
(intransitive) foll by in. to be convinced of the truth or existence (of): to believe in fairies
(intransitive) to have religious faith
(when transitive, takes a clause as object) to think, assume, or suppose: I believe that he has left already
(transitive; foll by of; used with can, could, would, etc) to think that someone is able to do (a particular action): I wouldn’t have believed it of him
n.
v.

believe it or not
believe one’s ears