credential n.

"letters entitling the bearer to certain credit or confidence," 1670s, from Medieval Latin credentialis, from credentia  "belief," from Latin credere "to believe, trust" (see credo). Probably immediately as a shortening of letters credential (1520s, with French word order); earlier was letter of credence (mid-14c.). Especially in reference to the letters of authorization given by a government to an ambassador.

early 13c., "the Creed in the Church service," from Latin credo "I believe," the first word of the Apostles' and Nicene creeds, first person singular present indicative of credere "to believe," from PIE compound *kerd-dhe- "to believe," literally "to put one's heart" (source also of Old Irish cretim, Irish creidim, Welsh credu "I believe," Sanskrit śrad-dhā- "faith, confidence, devotion"), from PIE root *kerd- "heart." The nativized form is creed. General sense of "formula or statement of belief" is from 1580s.