Hume believed there were two psychological mechanisms that led to religious feeling, which he called superstition and enthusiasm. Superstition is fear of the unknown, which leads to trust in priests and rituals to keep supernatural forces from harming one. Enthusiasm is a positive belief about the unknown, such as that God or gods are talking to me, or I am their special favorite.
Enthusiasm is dangerous in the short term — think Manson, or the Taiping rebels — but ultimately burns itself out. Superstition creeps in subtly and does long term damage as priests either deliberately or by gradual imperceptible evolution achieve positions of political power by exploiting superstition, promising the superstitious to allay their fears if they are obeyed.
Enthusiasm is love for what we do not understand and superstition is fear of what we do not understand. Writing in the eighteenth century Hume is especially concerned to limit the power of religious emotions to provoke cruelty, bolster oppressive regimes, and give rise to inquisitions. Yet while superstition seems unequivocally bad — a sort of learned helplessness that causes us to mistrust our ability to think our way through our own problems and to rely upon the unscrupulous or ignorant to allay our anxiety — enthusiasm can be good. The challenge is to figure out how to harness enthusiasm so it doesn’t burn us with an auto da fe or holy war, but doesn’t go out either.