To Florus 1.133

They boasted of observing ceremonies and sacrifices, and therefore supposed that other nations, consecrated by none of the Law's rites, neither could nor should suddenly be admitted to fellowship with them. He would tell them that even if the whole of righteousness had consisted in those observances, God would still have had power to make a change among the peoples, rejecting whom he wished and taking whom he wished. In answer to this meaning, a reply is made in the person of the Jews: nothing should now be demanded from the human will, since God “has mercy on whom he wills and hardens whom he wills” (Romans 9:18). To this the apostle replies: “O human being, who are you to answer back to God?” (Romans 9:20). He introduces the prophet Isaiah's testimony: “Does what is molded say to the one who molded it, ‘Why did you make me like this?’” (Isaiah 45:9; Romans 9:20). He adds in his own words: “Does the potter not have power over the clay to make from the same lump one vessel for honor and another for dishonor?” (Romans 9:21).

The meaning is this: “Because I commended God's will and explained the authority of his grace, saying that he grants mercy to whomever he has mercy on, you, O Jew, raised a false accusation against me. You acted as though my commendation of divine will and power involved the destruction of divine justice. Because I said, ‘He does what he wills’, you argued that nothing should any longer be demanded of human will if God does everything according to his own will. Yet the dignity of the person concerned left no room for that objection. If I had said, ‘God does what he ought, according to the laws of his justice which judges each person's deserts’, you certainly would not have raised the objection you now raise. But because I said, ‘God does what he wills’, you thought I had stolen something from justice's dignity. Both statements, then, mean the same thing. When I say of God, ‘He does what he wills’, I say nothing other than, ‘He does what he ought’, because I know that he wills nothing other than what he ought. Where will is inseparably joined to fairness, whichever of them I name, I have indicated both.”