To take something on faith means to believe it without good reason, so by definition a faith in the existence of supernatural entities clashes with reason. Religions also commonly clash with humanism whenever they elevate some moral good above the well-being of humans, such as accepting a divine savior, ratifying a sacred narrative, enforcing rituals and taboos, proselytizing other people to do the same, and punishing or demonizing those who don’t. Religions can also clash with humanism by valuing souls above lives, which is not as uplifting as it sounds.
