Credenda —Things to be believed
Credenda refers to a doctrine that must be believed by the Christian faithful as pertaining to the faith itself.

De fide — A matter of the faith
De fide are essential to the faith, and based in Revelation. A doctrine proposed de fide in an ex cathedra fashion is said to possess the highest degree of certainty of truth, and must be believed by the faithful.

De fide definita — Matter of the defined faith
De fide definita refers to a doctrine which is held to be formally defined by the Church’s Magisterium and therefore not open to denial, further speculation, or revision, and which is required to be held as an article of faith by all believers.