As a matter of act there was nothing in the Anglo-Saxon history akin to the classes of "slaves" in the Roman Empire. Anything other than a chattel-slave was called something else, e.g. "serf".

Yes, chattel slavery existed in the ancient world. No, the Church never taught that treating people as property was moral.

Paul, Ephesians:

5 Slaves, obey your earthly masters with respect and fear, and with sincerity of heart, just as you would obey Christ. 6 Obey them not only to win their favor when their eye is on you, but as slaves of Christ, doing the will of God from your heart. 7 Serve wholeheartedly, as if you were serving the Lord, not people, 8 because you know that the Lord will reward each one for whatever good they do, whether they are slave or free. 9 And masters, treat your slaves in the same way. Do not threaten them, since you know that he who is both their Master and yours is in heaven, and there is no favoritism with him.

The sin is exploitation, humans as tools.