20 “He stretches forth His hand in requiting”. “They have polluted His Testament.” Read the testament which they have polluted: “In your seed shall be blessed all nations.” Thou against these words of the Testator sayest what? The Africa of holy Donatus has alone deserved this grace, in him has remained the Church of Christ. Say at least the Church of Donatus. Wherefore do you add, of Christ? Of whom it is said, “In your seed shall be blessed all nations.” After Donatus will you go? Set aside Christ, and then secede. See therefore what follows: “They have polluted His Testament.” What Testament? To Abraham have been spoken the promises, and to his seed. The Apostle says, “Nevertheless, a man's testament confirmed no one makes void, or super-adds to: to Abraham have been spoken the promises, and to his seed. He says not, And to seeds, as if in many; but as if in one, And to your Seed, which is Christ.” In this Christ, therefore, what Testament has been promised? “In your seed shall be blessed all nations.” Thou that hast given up the unity of all nations, and in a part hast remained, hast polluted His Testament....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)