John Chrysostom

"And that the Gentiles might glorify God for His mercy." But what he means is this. Those of the Jews would have had promises, even though they were unworthy. But thou hadst not this even, but wast saved from love towards man alone, even if, to put it at the lowest, they too would not have been the better for the promises, unless Christ had come. But yet that he might amalgamate (or temper) them and not allow them to rise up against the weak, he makes mention of the promises. …

Homily on Romans 28 ↗

Theophylact of Ohrid

Those who came from the Jews, although they were unworthy, nevertheless had the promises; but you, called from paganism, were saved by lovingkindness alone. Therefore you are especially obligated to glorify God. And God is glorified when you live in union and unity among yourselves, when you bear with the weak.

Commentary on Romans ↗

Thomas Aquinas

Then when he says, but that the gentiles, he shows that even the gentiles were accepted by Christ: first, he states his proposition; second, he confirms it with an authority, at as it is written. First, therefore, he says: It has been stated that Christ accepted the Jews in order that the promises made to the patriarchs be fulfilled. But the promises were not made to the gentiles. Hence, this does not apply to the gentiles, because they were accepted on account of mercy. …

Commentary on Romans ↗