Convince people not to drag the rich fucks out into the streets and beat them to death because they promised the rich assholes will deffinitly get what they deserve in the totally real next life
Not purely of course. But as long as e.g. Islam exists, the extinction of slavery gets postponed.
Or other sects where e.g. women are an absolute property. There's a reason they're growing so much.
Ofc there's more slavery than that (US-prisons, US-wage-slavery, china etc), but those are things that could be resolved somehow. Religions can't. They're rarely open for discussions.