1) "And they that have believing masters" (hoi de pistous echontes) "And the ones believing, having despotic masters who believe." Those despotic masters refer to those who bought and, economically by right in the social order, owned the slaves, at a fixed market value, the same as cattle. Yet Christianity would have each to act as brother to the other.
2) " Let them not despise them " (me kataphroneitosan) "Let them not despise or lightly hold or respect them." The place of both master and slave was brought to a higher plane of respect for each other in society, through Jesus Christ; it did not set one against the other.
3) "Because they are brethren" (hoti adelpoi eisin) "Because brethren they are," in the new nature, on a spiritual plane. Yet Paul did not advocate anarchy against that social system or set master against slave and brother against brother. Galatians 3:28.
4) "But rather do them service" (alla mallon douleuetosan) "But rather let them serve as slaves," or let them do service for the Lord as slaves. No person is so low-born that he cannot serve Christ, Titus 2:9-10.
5) "Because they are faithful and beloved" (hoti pistoi eisin kai agapetoi) "Because faithful and beloved ones they are," the masters who are believers, Colossians 4:1; Ephesians 6:9.
6) "Partakers of the benefit" (oi tes euergesias antilambanomenoi) "The ones (masters) receiving in return good service." The slave-servant who became a Christian was to be a better servant to his master than ever before, and the Christian master was to be a better master to his slave-servants than before they became brethren; 1 Corinthians 7:20-22; Philemon 1:16-18.
7) "These things teach and exhort." (tauta didaske kai parakalei) "These kind of things teach thou and motivate," or, "exhort by calling alongside or counseling." The things referred to are social, ethical, and Christian moral responsibilities of slaves or servants to masters, and masters to servants, as employers and employees in Christ, 1 Corinthians 10:31.
THE SLAVE WINNING HIS MASTER FOR CHRIST
Many a heathen master was rebuked amid his career of profligacy by the saintly lives of Christian slaves who had given themselves up to the Lord of purity; and probably the hearts of many were touched through the prayers of those they had despised. We have read of a negress in the Southern states who was caught praying by her master and cruelly beaten for her pains. Stripped and tied fast to the post, as the blood-stained whip ceased for a moment to fall on the quivering flesh, she was asked if she would give over praying. "No, massa, never!!" was the answer; "I will serve you, but I must serve God." Again the lashes rained down on her bleeding back; but when once more they ceased, the voice of the follower of Jesus was heard praying, "0 Lord, forgive poor massa, and bless him." Suddenly the whip fell from his hand; stricken with the finger of God, he broke down in penitence. Then and there the prayer was answered -- the godless master was saved through the faithfulness of the slave he had despised.
-Gray & Adams Bible Commentary