It’s true. If you’re given a world of stupid people to rule over, a smart way to keep them under control is to create something akin to religion. They may not care enough about spending time in jail. Even the thought of being put to death wouldn’t stop some people from breaking your rules. But if they think they’ll be, say, tortured forever, they’ll probably think twice.
People can’t really torture people forever though. The torturers will get tired, their machines will malfunction, the torturees will eventually die or something else will happen to end it. Even if it never ended, the rule breaker would have to get caught first. So you have to create a “God” who knows everything and has the power to do anything. That way there’s no escaping.
But don’t just make God a punisher. You don’t want people simply living out of fear. So tell them that, while being bad will result in eternal bad for them, being good will result in eternal good. If they follow all “His” rules, they’ll be rewarded to an eternity in the best place imaginable; a place of happiness and bliss, where their dead loved ones will be waiting for them.
Smart people will demand proof for such a place, but don’t worry about them because they don’t exist in your world. Your people will believe anything you say because it comes from “God”. If they want to end their lives to get to God quicker, tell them suicide is a sin. You can’t have them offing themselves. You need them there to work for you and keep the population going.
If them having sex for other reasons goes against current social norms, tell them God says no sex before marriage and homosexuality is a sin. Are you a sexist male? Tell them women came from men. Are you a white racist? Tell them black slavery is fine. Put it all in a book, kick back and give yourself a pat on the back for making your job as ruler a lot easier.