There are actually 4 interesting scriptures in the NT were Paul mention's slavery:
Paul regarding his slave friend Onesimous:
14 But I did not want to do anything without your consent, so that any favor you do will be spontaneous and not forced. 15 Perhaps the reason he was separated from you for a little while was that you might have him back for good? 16 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 man and as a brother in the Lord.
Paul's request was to free this slave and to no longer treat him as a slave but as a dear brother. He also told the Galatians that from a Christian perspective there was neither slave nor free... for you are all one in Christ Jesus. In Corithians he lumped everybody together in "one body" .... slave or free - 12:13.
In colossians 3: 11 -
11 Here there is no Greek or Jew, circumcised or uncircumcised, barbarian, Scythian, slave or free, but Christ is all, and is in all.
To put how radical these statements might have seemed at the time. Greece ended up with about a 75% slave population and Rome had a more than 50% slave population. So, it was probably in the minds of citizens an absolute pre-requisite for civilization.
As a poor analogy, it might be like trying to tell someone today that we should abolish all trucks. We'll just need to get along with cars only. Think of the commercial losses that would cause. One could easily imagine the financial collapse of a system.
Aristotle called the institution of slavery , "natural" and "expedient" and "just". He said, "a slave is a living tool, just as a tool is a living slave, therefore there can be no friendship with a slave as slave".- Nichomachean Ethics 8:11 So, the practice was deeply engrained culturally, ethically, and financially and had been so for a very, very long time.
How many early Christians took Paul's seeds of abolition that he planted and grew them? We'll probably never know exactly. There were many though that understood Paul's words as being that slavery was incompatible with Christianity. Historian W. E. H Lecky says,
"St Melania was said to have emancipated 8000 slaves; St Ovidius a rich martyr of Gaul, 5000; Cromatius a Roman Prefect under Diocletian, 1400; Hermes a Prefect under Trajan 1200, [And] many of the Christian clergy at Hippo under the rule of St. Augistine, as well as great numbers of private individuals, freed their slaves as an act of piety". I'm sure they thought they were just being obedient.
I think it took courage for early Christians to free slaves because as Alvin Schmidt notes, "Roman edicts did not favor freeing slaves". This is understandable because there were some pretty big slave revolts at times in Roman history, Spartacus being my favorite. Nice movie renter.
In 315 Constantine imposed the death penalty on those that stole children to bring them up as slaves. Justinian, 527 - 565 AD abolished all laws that prevented the freeing of slaves to officialize what his fellow Christians had already been doing for a long time. Lactantius (the Christian Cicero) in his Divine Institutes said that in God's eyes there were no slaves. St. Augustine saw slavery as the product of sin and as contrary to God's divine plan - The City of God 19:15. St. Chrysostom in the 300's preached that when Christ came he annulled slavery. "Buy them, and after you have taught them some skill by which they can maintain themselves, set them free" . For five centuries the Trinitarian monks redeemed Christian slaves from servitude in Muslim occupied Spain. Lecky says that "in the thirteenth century there were no slaves to emancipate in France" and that in previous times there were "multitudes of them [slaves] embracing Christianity.
Church Historian Herbert Workman has shown that early Christians truly saw slaves as their brothers noting that no grave of a dead slave in the Christian catacombs was ever inscribed with the name "slave" . Callixtus, who was once a slave in the third century, became a priest, and the bishop of Rome and is later listed by the Roman Catholic Church as an early Pope.
In spite of Paul's words in the four instances in his writings. Many "Christians" all throughout this time owned slaves though... even prominent Church fathers. At times they even spoke approvingly of it. Popes even issued edits making it legal to own slaves for even the clergy. These erring Christians were sinning plain and simple. Either knowingly or not they were imitating the "world" around them and not growing into the type of people Christ was. The point is that Paul started people to feeling guilty about slavery, got them thinking about it, taught that they were equals to freedmen in God's eyes, and never said one word in support of the ancient institution. An honest reading shows that he was against it in my opinion.
The point is that Christ "came not to condemn the world but to save it". The NT is not a book of Laws but a book that shows the path to overcoming the need for laws by allowing Christ to live His life through you.
But there is two laws worth mentioning in the NT: Love God with your whole heart..... and your neighbor as yourself. That "law" was determined by Christians to include the practice of slavery.