Yes he was. When studying Paul’s letters, it is important to consider Paul’s views on important social and cultural institutions of Greco-Roman society. One of these institutions was slavery.
Slavery was an important part of the economy in the Roman empire, and the lives of most slaves were by no means easy. You can read about some of this online in Keith Bradley’s Resisting Slavery in Ancient Rome. Slaves were also integrated within social and family life, as slaves were considered to belong to the “household” as broadly understood in antiquity. They were objects owned by their masters and subject to the orders of their masters, but belonged to the “family” at the lowest rung in the ladder.
Slaves were also subject to punishment for failing to obey their masters, and this could sometimes be quite brutal, as the quotations from Galen and Seneca below indicate. It seems that Paul, like other contemporaries, assumed the continued existence of slavery and did not show any signs of calling for its abolishment or even for the manumission (setting free) of slaves. When Paul wrote a letter of recommendation on behalf of Onesimus, who was most likely a runaway slave, he did not ask Onesimus’ master, Philemon, to free (manumit) the slave. Nor did Paul call for the end of slavery. Elsewhere Paul advised that slaves (and others) should remain as they are in light of the present distress and coming end (1 Cor 7:21-24).
Paul, like virtually all of his contemporaries, could not imagine a society that did not have a system of slavery. Nonetheless, it may be that Paul, like some contemporary philosophers, did advocate that masters like Philemon at least treat their slaves in a more controlled manner, or even as a “brother”, as Paul puts it (at least if the slave belonged to the Jesus movement). In writing his letter, Paul seems to be concerned that Onesimus the slave not receive severe punishment from his master for whatever wrongdoing or disobedience his master perceived.
So Paul’s concerns may have something in common with the sentiments of upper-class authors such as Galen and Seneca. Galen, a physician and philosopher who lived in Pergamum (Asia Minor) in the second century, had this to say about punishing slaves:
“If a man adheres to the practice of never striking any of his slaves with his hand, he will be less likely to succumb [to a fit of anger] later on. . . my father trained me to behave in this way myself. . . . There are other people who don’t just hit their slaves, but kick them and gouge out their eyes. . . . The story is told that the Emperor Hadrian struck one of his attendants in the eye with a pen. When he realised that [the slave] had become blind in one eye as a result of this stroke, he called him to him and offered to let him ask him for any gift to make up for what he had suffered. When the victim remained silent, Hadrian again asked him to make a request of whatever he wanted. He declined to accept anything else, but asked for his eye back — for what gift could provide compensation for the loss of an eye?” (Galen, The Diseases of the Mind, 4; translation from T. Wiedemann, Greek and Roman Slavery [London: Croom Helm, 1981] 180-81).
Seneca, a first-century philosopher, stressed that one needed to control one’s passions or impulses in order to live a wise life (the philosophical life). In the context of discussing the control of anger, he used the treatment of slaves as an example:
“Why do I have to punish my slave with a whipping or imprisonment if he gives me a cheeky answer or disrespectful look or mutters something which I can’t quite hear? Is my status so special that offending my ears should be a crime? There are many people who have forgiven defeated enemies — am I not to forgive someone for being lazy or careless or talkative? If he’s a child, his age should excuse him, if female, her sex, if he doesn’t belong to me, his independence, and if he does belong to my household, the ties of family” (Seneca, Dialogue 5: On Anger, 3.24; translation from Wiedemann, Greek and Roman Slavery, 179-80).
Both Paul and Seneca seem to be concerned with modifying perceptions of status in some cases and with alleviating the negative treatments that could flow from status-distinctions, but neither had in mind an end to slavery.