Chattel slavery never really stopped during the Middle Ages, nor after Christianity became the official religion in the Roman Empire. It actually experienced a surge after the Roman Empire collapsed.

The main difference between classical and Christian views was that Christianity offered religious equality to everyone, including slaves. It still preached obedience to their masters and non-resistance to injustice, though. Later it was mostly replaced with serfdom (a different form of forced labor), and the Church itself became feudal.

Slavery experienced an another surge when the Atlantic slave trading began. Several popes condemned it, but it wasn’t effective. It took the Enlightenment culture to end it.