Or, "God is one" presents the thought that there is no other God to whom the Gentiles could turn for justification. There is one God, and from Him alone justification can proceed.
If it cannot be thought that God abandoned the Gentiles to destruction, then it must be accepted that He gives justification to them also. If God is one, and all people are of one nature, then the condition of justification must be one for all.
That condition is faith. By faith both the circumcised and the uncircumcised are justified. "There is one God of all, one Creator of all. It is impossible that He should care for some and leave others without care.
"Consequently, He gives salvation to all who believe" (blessed Theodoret).
It is true that the Apostle assigns justification to the circumcised ἐκ πίστεως, from faith or because of faith, and to the uncircumcised διὰ τῆς πίστεως, through faith.
Yet one cannot say precisely what nuance of thought about the justification of each group the Apostle wished to express by this. It seems he wished only to distinguish the justification of both, but by one undivided faith.
The power of this faith is the same for both. He equally brings all under the shelter of faith. There is, of course, a certain difference between Jew and Gentile.
For the Jew, both the Gospel and faith had been sown as seed in the law and prophetic Scriptures. He was already within God's enclosure.
The Gentile, however, could rise only to the feeling that salvation was impossible except by God's special help. He could also feel that God would not abandon him, since He is God of all.
Yet he still had to enter God's enclosure through some door. For him this door is faith. Through faith, διὰ πίστεως, he enters.
For the Jew it remained only to be sprinkled from the fountain of faith, ἐκ πίστεως, the prompting of which he could draw from the law and the Scriptures.
The fruit of this is one and the same for both: justification. Is this not what the holy Apostle hints at here?