since God is one—who will justify the circumcised by faith and the uncircumcised through faith.
30. Who shall justify, The future is used for the present — “who justifies,” after the manner of the Hebrew language, though some consider that the day of judgment is referred to; but he seems to speak of a present act, or as Grotius says, of a continued act, which the Hebrews expressed by the future tense. — Ed. etc.
From all this Paul infers that it is in vain to look for justification by the works of the law, and that it is to be had only by faith, which is the point he has been all along proving, from Rom 1:17, and which he lays down (Rom 3:28) as the summary of his discourse, with a quod erat demonstrandum - which was to be demonstrated.
Commenting on Romans 3:19-31
Verse 30. It is one God. The same God; there is but one, and his plan is equally fitted to Jews and Gentiles. The circumcision. Those who are circumcised--the Jews. The uncircumcision. Gentiles; all who were not Jews. By faith--through faith. There is no difference in the meaning of these expressions. Both denote that faith is the instrumental cause of justification, or acceptance with God. (e) "which shall justify"