mid-13c., "duty of fulfilling one's trust," from O.Fr. feid, foi "faith, belief, trust, confidence, pledge," from L. fides "trust, faith, confidence, reliance, credence, belief," from root of fidere "to trust," from PIE base *bheidh- (cf. Gk. pistis; see bid). For sense evolution, see belief. Theological sense is from late 14c.; religions called faiths since c.1300.