There are few emotions, if any, more powerful than fear. If it is made clear to the masses that if they fail to behave in the way authority tells them to, they will suffer eternal damnation and everlasting pain, but if they do as they are told, it will be all harps and angels. Suffer now on earth, starve, work until your back breaks, obey the rules, die young, see your children die of hunger, and eventually enter eternal glory. Disobey your priest and you are condemned. Must have been pretty hard to be an ignorant, illiterate, impoverished peasant with not much chance of standing up to a Church full of rich, massively powerful men. Must have been pretty hard for women who were easily and casually blamed for their own rape as adulterers (and still are) as well as witchcraft and other female sins. All Eve's fault, of course. God and the Devil are man-made (man, specifically) inventions designed to control the populace and ensure that the rich and powerful could retain their wealth, control and comfort.