felix, fido

Though the Urban Dictionary may yield a more colourful definition, someone described as fidious takes something on faith alone. One can find this element in the word perfidious, which jaunts off in rather the opposite direction to connote treachery or faithlessness, although the latter’s antonym is unperfidious—something that is not considered a breach of faith. A perfervour, again drifting off toward the other course is a zealous, patriotic individual—wholly fervid, heated and impassioned. Fidiousness is about belief and reliability, however, and not confidence and would probably be best opposed to a sceptical attitude.