‘If so be that God is one, and he will reckon the circumcision as in the right by faith (out of faith), and the uncircumcision through faith.'
And the grounds for his confident answer is that God is one. This was indeed what the Jew boasted about constantly, ‘YHWH our God, YHWH is one' (Deuteronomy 6:4). Well, says Paul, if He is One then He is God over all and will deal with all on the same terms. He will reckon the circumcision to be in the right by faith, and the uncircumcision to be right through faith. All will be dealt with in the same way.
This fact that God is God of both Jew and Gentile will be emphasised in the next passage where Paul calls on the example of Abraham, ‘the father of many nations'. He is thus here preparing the way for that thesis.
It may be asked whether we should distinguish ‘out of faith' from ‘through faith'. If there is a distinction it probably lies in the idea that the Jews were reckoned as right ‘by faith', and the Gentiles ‘through the same type of faith'. But the distinction is probably not intended to be seen as important.