Rulers were appointed kings by "divine right", and peasants were told "the meek shall inherit the Earth".
Dissenters, if individuals, were kept in line by threat of being burned alive at the stake. Dissenters, if rulers, faced the threat of the church fomenting revolution among their subjects by excommunication. The church eventually came to own more land than any single country or empire.
The power of the Church was only broken when the printing press broke the Church's monopoly on the distribution of the written word (the incredibly courageous actions of Martin Luther in defying the church helped a lot). This led directly to the flowering of art and science we call the Renaissance.
The insidious fatal flaw of religion, the facet that gives it so much power and allows it to do so much damage, is the notion of "divinely" inspired, irrefutable, faith based truth.
MY creed is correct, YOURS is wrong, not because my creed makes more sense, but because God has told me so. Or, more often, because some priest claims God has told him so, but I don't dare defy the priest. Don't bother trying to explain how wrong-headed I am,
God is by definition never wrong, and even implying otherwise is not only sinful in the eyes of God, but more significantly, heretical in the eyes of the church. God may not see fit to punish you, but a human religious authority is likely to be far less forgiving.
I strongly feel society is much better off when individuals are encouraged to think for themselves, to question authority, and to discuss their unconventional ideas with others. To me this is how progress occurs. Encouraging the notion that some authority, religious or otherwise, has all the answers, and that questioning that authority is in itself evil, leads to stagnation and repression.