John Calvin Commentary Romans 3:30

"if so be that God is one, and he shall justify the circumcision by faith, and the uncircumcision through faith." — Romans 3:30 (ASV)

Who shall justify, etc. In saying that some are justified by faith and others through faith, he seems to have taken the liberty of varying his language while expressing the same thing, and for this purpose: that he might, in passing, touch on the folly of the Jews, who imagined a difference between themselves and the Gentiles, though on the subject of justification there was no difference whatever. For since people became partakers of this grace by faith only, and since faith is the same in all, it is absurd to make a distinction in what is so alike.

I am therefore led to think that there is something ironical in the words, as though he said: “If anyone wishes to have a difference made between the Gentile and the Jew, let him take this—that the one obtains righteousness by faith, and the other through faith.”

However, it may be that some will prefer this distinction: that the Jews were justified by faith because they were born heirs of grace, as the right of adoption was transmitted to them from their ancestors; and that the Gentiles were justified through faith because the covenant was acquired by them.