If indeed ( εἴπερ rather than ἐπείπερ, as in the Textus Receptus) God is one, who shall justify the circumcision by faith, and the uncircumcision through faith. Here the unity of God is given as the reason of his being the God of the Gentiles as well as of the Jews.
So also, 1 Timothy 2:5, εἷς γὰρ θεὸς is the reason why he wills all men to be saved. It is of importance to grasp St. Paul's idea in his assertions of the unity of God. It is not that of numerical unity, but what may be called the unity of quality; i.
e. not a mere assertion of monotheism as against polytheism, but that the one God is one and the same to all, comprehending all in the embrace of his own essential unity. God's unity involved in St. Paul's mind the idea of "One God, the Father, of whom are all things, and we unto him" (1 Corinthians 8:6); "who made of one blood every nation of men" (Acts 17:26); in whom we (all of us) "live and move and have our being" (Acts 17:28).
Thus exclusion of the Gentiles from the paternal embrace of the one God is incompatible with the very idea, so conceived, of his unity. In the latter part of this verse it is said that God will justify the circumcision ἐκ πίστεως, and the uncircumcision διὰ τῆς πίστεως, the preposition being changed, and the second πίστεως being preceded by the article.
The difference is not of essential importance, "faith" being the emphatic word. But it is not unmeaning. ἐκ expresses the principle of justification; διὰ, the medium through which it may be had. The Jew was already in a position for justification through the Law leading up to Christ.
He had only to accept it as of faith, and not of works of law (verse 20). The Gentile must attain to it through faith; i.e. his faith in the gospel now revealed to him. ἐπὶ τῶν ἰουδαίων τὸ ἀκ πίστεως τέθεικεν ὡς ἂν ἐγόντων μὲν καὶ ἑτέρας ἀφορμὰς πρὸς δικαίωσιν, πίστεως" (Theodorus).