Why is the ninth month called September?
late 12c., from Latin, literally "I believe," first word of the Apostles' and Nicene Creeds, first person singular present indicative of credere "to believe," perhaps from PIE compound *kerd-dhe- "to believe," literally "to put one's heart" (cf. Old Irish cretim, Irish creidim, Welsh credu "I believe," Sanskrit śrad-dhā- "faith"). The nativized form is creed. General sense of "formula or statement of belief" is from 1580s.