3 In the presence of so much suffering, the condition of slavery, in which a considerable part of the great human family has been sunk in squalor and affliction now for many centuries, is deeply to be deplored; for the system is one which is wholly opposed to that which was originally ordained by God and by nature. The Supreme Author of all things so decreed that man should exercise a sort of royal dominion over beasts and cattle and fish and fowl, but never that men should exercise a like dominion over their fellow men. As St. Augustine puts it: "Having created man a reasonable being, and after His own likeness, God wished that he should rule only over the brute creation; that he should be the master, not of men, but of beasts." From this it follows that "the state of slavery is rightly regarded as a penalty upon the sinner; thus, the word slave does not occur in the Bible until the just man Noe branded with it the sin of his son. It was sin, therefore, which deserved this name; it was not natural."(4)
Source: In Plurimis (Vatican.va)