As also mentioned by Kirby Page in Jesus or Christianity, Charles Loring Brace tells us in Gesta Christ, or, A History of Humane Progress under Christianity (1888) that it was not until the 9th century that the first recorded stand against slavery itself was taken by St. Theodore the Studite:
No direct word against slavery, however, came forth from the great Teacher [Jesus Christ]. It was not until the ninth century after, that one of his humble followers, Saint Theodore of Studium (Constantinople), ventured to put forth the command "Thou shalt possess no slave, neither for domestic service nor for the labor of the fields, for man is made in the image of God."
This quote is taken from the Testament (PG 99:1816D; 1817, ch. d) of St. Theodore, where he says that monasteries must not own slaves or female animals. He takes these prohibitions from his uncle St. Plato, the abbot of Sakkudion (Oration 11, PG 99:824-825, ch. 4.23). St. Theodore also mentions the subject of slavery in a letter to his student Nicholas (Epistle 10) and in his Catechesis (13). And in both of his recorded Lives, St. Theodore frees his slaves.
It should be noted that other Church Fathers before St. Theodore denounced slavery, though tolerated it at the secular level. For example, St. John Chrysostom examines the passions behind slavery, but explains that a faithful and godly slave is essentially free while those who are led by their passions and desires to sin are the true slaves. He writes: "Covetousness begat slavery, and an evil temper, and insatiable greediness. For Noah had no servant, neither had Abel, nor Seth, no, nor they who came after them. The thing was the fruit of sin, of rebellion against parents" (Homily on Ephesians 22). What differentiates St. Theodore's denunciation is that he does not tolerate it on the grounds that "man is made in the image of God", which would become the slogan for all later denunciations against slavery.
Read also: The Church Fathers and Slavery