Latin credere means “to trust” Used with the dative, it means “to believe, to give credence to a person or thing.” Several English words come from this word.
Credo with a capital refers to the Christian Apostles’ Creed or the Nicene Creed. Lowercase credo means any statement of the aims or principles which guide a person’s conduct.
Creed is used more often than credo for this declaration of guiding principles. For example, “The Declaration of Independence contains the clearest, most concise, and most eloquent articulation of the American creed.”
The noun credit came into English from Middle French with the meaning “belief, faith, trust.” It came to be associated with money lent or borrowed with an agreement for repayment. A customer’s “credit” is the confidence a lender has in the customer’s ability to repay. Credit is also used as a verb in the general sense of accepting something as true or truthful. For example, “He found it hard to credit his own eyes; the lion was lying down beside the lamb.”
Schools are “accredited” by outside regulating boards. They have received credentials stating that they can be trusted to meet certain standards.
Article IV, Section 1 of the United States Constitution is known as the “Full Faith and Credit Clause”:
Full faith and credit shall be given in each state to the public acts, records, and judicial proceedings of every other state. And the Congress may by general laws prescribe the manner in which such acts, records, and proceedings shall be proved, and the effect thereof.
The adjective credulous came into the language with the specific meaning “believing in God.” The negative incredulous meant “not believing in God.” The meaning shifted as time went on. Shakespeare used credulous in the sense of credible, but modern usage assigns distinct meaning to the two words.
Credulous has the connotation of being too ready to believe. It’s a synonym for gullible. For example, confidence tricksters prey upon credulous people who are quick to believe in get-rich-quick schemes.
Credible means “able to be believed.” A credible witness is one who can be trusted to speak the truth.
The negative forms, incredible and incredulous, are sometimes confused. For example, here’s a headline from a site selling a sweatshirt that bears a message about not touching the wearer:
incredulous dont [sic] touch me pullover sweatshirt
Like awesome, incredible is often used as a throwaway word to convey enthusiasm. Sweatshirts are incredible. Singers are incredible. Movies are incredible.
Generally speaking, incredible usually applies to things that are hard to believe, while credulous describes people who believe things too easily.