He notes that no philosopher of Sumer, Babylon, or Assyria "ever protested against slavery" or expressed any sympathy for its victims.
Plato saw slaves as "lacking the mental capacity for virtue and culture" p. 326). At his death, Plato’s estate included five slaves.
Aristotle justified slavery since slaves were "more akin to brute beasts than to free men" (p. 327). "Upon his death, Aristotle’s personal property included fourteen slaves" (p. 327).
Opposition to slavery developed only among Jews (the Essene and the Therapeutae sects) and Christians.
What about men of the Enlightenment? Surely these noble humanists opposed slavery! Think again, Stark says (see p. 359-60). Stark notes that "a virtual Who’s Who of ‘Enlightenment figures fully accepted slavery" (p. 359). These included Thomas Hobbes, John Locke (he invested in the African slave trade), and Voltair (who supported the slave trade and believed in the inferiority of Africans). We can add to our list of humanist slavery supporters Baron Montesquieu, Compte de Mirabeau, and Edmund Burke (he saw abolitionists as "religious fanatics").
Stark concludes: "It was not philosophers or secular humanists who assembled the moral indictment of slavery, but the very people they held in such contempt: men and women having intense Christian faith, who opposed slavery because it was sin" (p. 360).
The end of slavery in the West did not come through the efforts of humanists but from Quakers in North America, the preaching of the aged John Wesley, and the efforts of evangelical churchmen like William Wilberforce and the "Clapham Sect" in Great Britain.