Of the 13 letters attributed to Paul, seven are authentic by scholarly consensus. Three are disputed, probably not Paul. Three are certainly not Paul. Crossan labels them in order, radical, liberal, conservative (reactionary):
The seven authentic, certainly Paul letters (Radical)
I Corinthians (except I Corinthians 14:33b-36)
The three disputed letters, probably not Paul (liberal):
The three certainly not Paul, even anti-Paul (conservative/reactionary):
To illustrate, here is a passage from the authentic Paul (Galatians 3:28)
28There is no longer Jew or Greek, there is no longer slave or free, there is no longer male and female; for all of you are one in Christ Jesus.Paul's letter to Philemon, the slave-holder of Onesimus, is another authentic letter. Paul exhorts Philemon to take back his slave as a free person. For Paul, the authentic Paul that is, slavery is an anathema to oneness in Christ. For Paul, to be a Christian is to have no slaves. That is the radical Paul.
When we move to the disputed letters (probably not) of Paul, we have a different take on slavery. We are moving from the radical egalitarian message of Paul (no slavery in Christ) to a compromise with culture (in which slavery was an accepted part of Roman culture). Here are the texts, first from Ephesians:
Slaves, obey your earthly masters with fear and trembling, in singleness of heart, as you obey Christ; 6not only while being watched, and in order to please them, but as slaves of Christ, doing the will of God from the heart. 7Render service with enthusiasm, as to the Lord and not to men and women, 8knowing that whatever good we do, we will receive the same again from the Lord, whether we are slaves or free. 9 And, masters, do the same to them. Stop threatening them, for you know that both of you have the same Master in heaven, and with him there is no partiality. (Ephesians 6:5-9)And from Colossians:
Slaves, obey your earthly masters in everything, not only while being watched and in order to please them, but wholeheartedly, fearing the Lord. 23Whatever your task, put yourselves into it, as done for the Lord and not for your masters, 24since you know that from the Lord you will receive the inheritance as your reward; you serve the Lord Christ. 25For the wrongdoer will be paid back for whatever wrong has been done, and there is no partiality. 41Masters, treat your slaves justly and fairly, for you know that you also have a Master in heaven. (Colossians 3:22-4:1)
This is the liberal Paul, or the authentic Paul liberalized by the culture. This is not Paul, but people writing in his name. Here "Paul" accepts slavery but there is mutuality. There are responsibilities for slaves and masters. "Paul" does not do away with slavery, however, or suggest that you cannot be a Christian and own slaves. He suggests that there are responsibilities for both slaves and masters.
Finally, the conservative or reactionary Paul. This is from Titus:
9 Tell slaves to be submissive to their masters and to give satisfaction in every respect; they are not to answer back, 10not to pilfer, but to show complete and perfect fidelity, so that in everything they may be an ornament to the doctrine of God our Savior. (Titus 2:9-10)
This is not Paul either, but an imposter writing in his name. There is certainly no condemnation of slavery, nor is there even mutuality (own slaves but treat them with respect). In Titus, the admonition is to slaves alone with no admonition to the masters.
Finally, the historical Paul of the authentic letter to Philemon is not speaking about general slavery outside the Christian community. He is not speaking about pagan owners with pagan slaves. He is not talking about Christian owners with pagan slaves, or about pagan masters with Christian slaves. In that last case, he advises Christian slaves (in I Corinthians 7:21) that, if liberation is forthcoming, they should use their newfound freedom for Christ's advantage. Nevertheleess, despite Paul's very specific focus on Christian owners and Christian slaves, his belief that all people should be Christian would inevitably have included the belief that all should be free and equal. (p. 165)
In other words, to be Christian is to call slavery an anathema. You cannot own slaves if you are Christian. The radical historical Paul within the texts of the New Testament itself, by works attributed to him over time, is distorted and his message of radical egalitarianism eventually leads to compliance with the oppression and culture of Empire.
This is one reason why the historical/critical method of interpreting scripture is important. Paul and women next time...