"disposed to believe, uncritical with regard to beliefs," 1570s, from Latin credulus "that easily believes, trustful," from credere "to believe" (see credo). Related: Credulously; credulousness.
Alas, my Friends, credulous incredulity is a strange matter. But when a whole Nation is smitten with Suspicion, ... what help is there? Such Nation is already a mere hypochondriac bundle of diseases; as good as changed into glass; atrabiliar, decadent; and will suffer crises. Is not Suspicion itself the one thing to be suspected, as Montaigne feared only fear? [Carlyle, "French Revolution"]