I’d elaborate a bit on my interpretation of what the fella said.
The religion in point - catholicism, and maybe we can generalize to all abrahamic religions, I’m not very familiar with other religions to speak of them, instill a way of thinking that doing wrong is all fine and well as long as you repent and ask for forgiveness. Sound sensible, right? Except we’re dealing with people here so they take it to mean that you can do all sorts of crap as long as you say you’re sorry. It got so bad at some point that the pope was selling indulgences. ‘Give me money and I’ll let you sin’.
They also instill a sort of moral superiority on the adherents to said religions versus the pagans.
So yeah, slavery is worse (and I’m counting human trafficking here as well - it’s the modern version), but is it not facilitated by the mindset instilled by religion? First - you see them as savages needing to be civilized - that’s the moral superiority talking - you enslave them, BUT you bring them to god as well, so there’s a load off your moral issues. Add to that the fact that even if you were wrong and did bad stuff, you didn’t ‘know’ any better, and it’s ok cause hellfire won’t get you because you repent, there’s your free ticket.
On the other hand, if you kidnap and force good christians into sexual slavery, you can be pretty sure that you most likely won’t get murdered / maimed while you’re raping because their moral teachings say to turn the other cheek instead of fighting back. And one of the 10 comandments is thou shalt not kill. Also a belief in sky-papa dishing out punishment in the afterlife makes people less inclined to seek vengeance (compounded with the previous point - thou shalt submit to being dehumanized by a fellow human without recourse).
This is an oversimplification to make a point, but sure, religion is seemingly not worse than other crap people are capable of but it sure sets the groundwork nicely. Sort of like you need to know a language before you can swear in it. A tool, but less like a hammer and more like a scythe. One good use, but so many other bad ones.