I am profoundly confused by these two roots. Why should I not kredi someone who is not fidinda, why do people fidas in God, but things are nekredeblaj? What question should I pose in my mind when deciding which root to use in a word I'm going to say? Thank you!
Update: Turns out I was wrong about the phrase kredi je Dio, I don't know why I thought it was fidi. That certainly added needlessly to the confusion.
The primary meaning of kredi is believe (a statement). The primary meaning of fidi is trust (a person). So the original distinction is between putting faith in an idea, or in a person.
For example, kredi je Dio is to believe in God, i.e. to believe the statement "God exists." Fidi Dion is to trust God.
Secondary meanings allow you to kredi al persono, al viaj okuloj (to believe a person, i.e. to believe the statements of a person; to believe your eyes) and to fidi la vorton, la honestecon, la kapablojn de iu (to trust the word, the honesty, the abilities of a person).
Thus, in both these meanings, the difference is that kredi refers to the acceptance of specific statements or ideas, whereas fidi refers to a more general trust.
Kredo has to do with things that you hold to be true. Fido has to do with trust or confidence in something. It can also mean a "belief based on religion."
On this last point, I will call your attention to the following clarifying note from PIV:
En tiuj ekzemploj fido aspektas kiel sinonimo de kredo. Sed la jena ekzemplo montras la diferencon: ankaŭ la diablo kredas je Dio, sed mankas al li la ĝusta fido
Mi kredas je Dio = I believe in God (I believe that she exists)
Mi fidas je Dio = I trust in God (I trust what she says)
