Believe in

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).
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
v.

believe it or not
believe one’s ears