John Chrysostom

This applies to the other, that he may grow peaceable. But the other to the latter too, that he may not destroy his brother. Still he has made both apply to either again, by saying, "one another," and showing that without peace it is not easy to edify.

Homily on Romans 26 ↗

Theophylact of Ohrid

He said "of peace" to the imperfect one, because that one did not have peace, and "for mutual edification" he said to the perfect one, so that he would not unsettle his brother by scandalizing him. However, by saying "mutual," he directed his word about this to both the one and the other. Moreover, by mentioning peace first and then edification, he showed that without peace it is difficult to edify.

Commentary on Romans ↗

Thomas Aquinas

Then when he says, therefore, he infers the intended admonition. Inasmuch as the kingdom of God consists in justice, peace, and spiritual joy, therefore, in order to arrive at the kingdom of God, let us follow after the path of peace, i.e., strive to accomplish the things through which we conserve the peace of Christians: strive for peace and holiness (Heb 12:14). …

Commentary on Romans ↗