The root "cred" means believe. These are vocabulary words stemming from that root.
accreditation
granting approval or belief in a school
credential
a document that proves a person is believable
credible
believable; reliable
credit
to believe that someone will do something
creditor
a person who believes that he will be paid back the money that he loaned
credulous
tending to believe too easily; easily convinced; easily fooled
creed
a set of religious beliefs or principles
discredit
to refuse to believe; to reject as untrue
incredible
not believable; improbable; unlikely
incredulous
doubting; unwilling or unable to believe