You think Pauline Christianity is about conforming to reality, not transcending it by God's sending his Son to defeat the devil and to save us with his sacrificial death? Paul added Gnosticism to Christianity, not so much Stoicism. Stoicism is a kind of secular, philosophical ethics. The New Testament is roundly anti-philosophical.

The religions of the ancient world did conform to slavery, though, in that they rationalized the practice, and that includes the Bible. Some Pauline epistles tell slaves to obey their masters (Eph. 6:5, Col. 3:22).

Also, Paul didn't appeal to the virtue of "justice" in Philemon. He appeals, rather, to cultic Christian brotherhood: "no longer as a slave, but better than a slave, as a dear brother. He is very dear to me but even dearer to you, both as a fellow man and as a brother in the Lord" (1:16).