Most of the west and north European flavor of Christianity originated from Irish monks if I remember correctly. As they spread their understanding of Christianity, slavery pretty much vanished in those parts of the world. It made it pretty much impossible to enslave believers of the one true god because they are your brothers. It also conveniently served as a convincing argument to convert so it definitely wasn’t all rosy either.

The Byzantine Empire on the other hand had a different flavor of Christianity going back from Roman times where slavery was fundamental to society. Eventually Byzantine fell and many Europeans believed it was a punishment from god for their debauchery and mistreatment of fellow Christians as slaves.

Slavery became much more common again after the age of enlightenment and absolutism in early modern times when European monarchies really started colonizing the globe on an industrial scale. Most of Christian Europe were absolutely against it for almost a thousand years. So yeah, it’s totally understandable people in 1500 thought it was wrong.

I believe many people like to think slavery is just a default for civilization and abolishing it was a fairly recent idea but there always were cultures, places, and periods in history where it wasn’t a thing because people aren’t inherently cruel and selfish. Many people in history knew slavery was wrong.