John Calvin on Romans 9:21

21. Has not the worker of the clay? etc. The reason why what is formed ought not to contend with its former, is, that the former does nothing but what he has a right to do. By the word power, he means not that the maker has strength to do according to his will, but that this privilege rightly and justly belongs to him. For he intends not to claim for God any arbitrary power but what ought to be justly ascribed to him.

And further, bear this in mind, — that as the potter takes away nothing from the clay, whatever form he may give it; so God takes away nothing from man, in whatever condition he may create him. Only this is to be remembered, that God is deprived of a portion of his honor, except such an authority over men be conceded to him as to constitute him the arbitrator of life and death. The metaphor in these verses is doubtless to be interpreted according to the context. Not only Calvin, but many others, have deduced from it what is not consistent with what the next verse contains, which gives the necessary explanation. By the “mass” or the lump of clay, is not meant mankind, contemplated as creatures, but as fallen creatures; or, as Augustine and Pareus call them, “massa damnata — the condemned mass;” for they are called in the next verse vessels of wrath, that is, the objects of wrath; and such are all by nature, according to what Paul says in Ephesians 2:3; “we were,” he says, “by nature the children of wrath, even as others.”

“The words, ‘I will have mercy on whom I will have mercy,’ imply that all deserved wrath; so that the lump of clay in the hands of the potter must refer to men already existing in God’s foreknowledge as fallen creatures.” — Scott

In all the instances in which this metaphor is used by Isaiah and Jeremiah, it is applied to the Jews in their state of degeneracy, and very pointedly in Isaiah 64:8: where it is preceded, in the 6th verse, by that remarkable passage, “We are all as an unclean thing,” etc. The clay then, or the mass, is the mass of mankind as corrupted and depraved. — Ed.

Romans 9:22-23

22. What if God, willing to show his wrath, and to make his power known, endured with much long-suffering the vessels of wrath fitted to destruction:

22. Quid autem si Deus volens demonstrare iram, et notam facere potentiam suam, sustinuit in multa patientia vasa irae, in interitum ap-parata;

23. And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory,

23. Ut notas quoque faceret divi-tins gloriae sum in vasa misericordiae, quae preparavit in gloriam?