John Chrysostom

"Among whom are ye also the called of Jesus Christ." That is, along with whom ye also are: and he does not say, that he called the others with you, but you with the others. For if in Christ Jesus there is neither bond nor free, much less is there king and private man. For even ye were called and did not come over of yourselves.

Homily on Romans 1 ↗

Theophylact of Ohrid

Here he crushes the arrogance of the Romans. You have not received more than the other nations, over whom you consider yourselves masters; therefore, just as we preach to the other nations, so also to you: do not boast. Otherwise: you too were called, you were preceded by grace, and did not come on your own.

Commentary on Romans ↗

Thomas Aquinas

Fourth, as to his power over those to whom he was writing and who were subject to his apostleship. Hence he says among whom, i.e., I number among those subject to my apostolate even you Romans howsoever lofty: he lays it low, the lofty city, the foot of the poor, i.e., of Christ, tramples it, the steps of the needy, namely, of the apostles Peter and Paul (Isa 26:5ff.); we were the first to come all the way to you with the Gospel of Christ (2 Cor 10:14). …

Commentary on Romans ↗