Slaves and masters
Household instructions to slaves and masters (also 1 Peter 2:18-21 and Philemon) beside 'neither slave nor free' — regulatory, transformative and abolitionist readings of the institution.
Ephesians 6:5-9
5Servants, be obedient to those who according to the flesh are your masters, with fear and trembling, in singleness of your heart, as to Christ,
6not in the way of service only when eyes are on you, as men pleasers, but as servants of Christ, doing the will of God from the heart,
7with good will doing service as to the Lord, and not to men,
8knowing that whatever good thing each one does, he will receive the same good again from the Lord, whether he is bound or free.
9You masters, do the same things to them, and give up threatening, knowing that he who is both their Master and yours is in heaven, and there is no partiality with him.
Galatians 3:28
28There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus.
What each tradition says
Rabbinic / Midrashic
According to the Jewish Encyclopedia, bondmen and bondwomen are at least a subordinate part of the Jewish community: males are circumcised and both sexes received into the fold. Selling a bondman or bondwoman to a Gentile is forbidden, since they might be driven into apostasy; a transfer of services briefly, or with Sabbaths and festivals reserved, is perhaps lawful.
Wilhelm Bacher, Lewis N. Dembitz, Gotthard Deutsch and Samuel Krauss (The Jewish Encyclopedia), The Jewish Encyclopedia, vol. 11, s.v. 'Slaves and Slavery' (New York: Funk & Wagnalls, 1901-1906) — section 'Canaanite Bondmen of the Talmud'
Patristic
Augustine says that by nature, as God first created us, no one is a slave of man or sin. This servitude is penal, appointed by the law that preserves the natural order and forbids its disturbance. If nothing had violated that law, there would be nothing to restrain by penal servitude.
Augustine of Hippo (354-430), The City of God, Book XIX (trans. Marcus Dods), in Nicene and Post-Nicene Fathers, ser. 1, vol. 2 (New York: Christian Literature Publishing Co., 1890) — Book XIX, ch. 15 ('Of the Freedom of the Will...')
Catholic
James Fox states that from the beginning Christian moralists did not condemn slavery as essentially against natural law or natural justice. Since slavery, tempered by many humane restrictions, was permitted under Mosaic law, that would have sufficed to keep Christian teachers from condemning the institution as absolutely immoral.
James Fox (The Catholic Encyclopedia, 1912), The Catholic Encyclopedia, vol. 14, s.v. 'Ethical Aspect of Slavery' (New York: Robert Appleton Company, 1912) — opening section (thesis, on de Lugo and Gerdil)
Orthodox
Chrysostom emphasizes that a slave has become a brother, enjoys the same privileges, belongs to the same body, and is even a brother of the Son of God. Yet, he observes, this same person is told, "obey your masters according to the flesh, with fear and trembling."
John Chrysostom (c. 349-407), Homilies on the Epistle to the Ephesians (trans. Gross Alexander), in Nicene and Post-Nicene Fathers, ser. 1, vol. 13 (New York: Christian Literature Publishing Co., 1889) — Homily XXII on Ephesians 6:5-8
Reformed / Protestant
Calvin observes that no royal edicts had ever been issued to protect slaves. Yet he says God allows masters no power over them beyond what is consistent with the law of love.
John Calvin (1509-1564), Commentaries on the Epistles of Paul the Apostle to the Galatians and Ephesians (trans. William Pringle, Calvin Translation Society, 1854) — Commentary on Ephesians 6:9
Evangelical
John Wesley states that liberty is the right of every human creature as soon as he breathes the vital air. No human law can deprive him of that right, which he derives from the law of nature.
John Wesley (1703-1791), Thoughts upon Slavery (London: Printed by R. Hawes, 1774; Project Gutenberg etext 68144) — §6 (address to slaveholders)
Evangelical
Barnes writes that it is not certain what kind of servant he was. The word does not necessarily mean slave, and neither this passage nor any other part of the epistle proves that he was ever a slave.
Albert Barnes (1798-1870), Notes on the New Testament, Explanatory and Practical (enlarged ed. by Robert Frew; CCEL carries the Baker reprint) — Note on Philemon 16
Evangelical
Thornton Stringfellow holds that Paul’s letter to Philemon is by itself sufficient to teach any ordinarily teachable person. It shows that slavery, in God’s sight, is not what it is in the sight of abolitionists.
Thornton Stringfellow (1788-1869), Scriptural and Statistical Views in Favor of Slavery (Richmond, Va.: J. W. Randolph, 1856) — section on Paul's letter to Philemon