John Chrysostom
Earlier the Apostle said, "inasmuch as I am an Apostle to the Gentiles, I magnify my ministry" (Rom 11:13). Likewise, "lest perhaps He also spare not you" (Rom 11:21). Again, "be not wise in your own conceits" (Rom 12:16). Then, "why do you judge your brother?" (Rom 14:10). Again, "who are you, judging another’s servant?" (Rom 14:4). He also used many other similar expressions.
After he had said much that was severe in his Epistle, in conclusion he heals the wounds he had inflicted. He ends as he began.
At the beginning he said, "I thank my God for you all, because your faith is proclaimed in all the world" (Rom 1:8). Here he says, "I myself am persuaded that you are full of goodness, able also to admonish others." In this place he says even more than in the first.
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He did not say, "I have heard," but, "I am persuaded." That is, I have no need to learn from another, but I myself am confident about you, I who rebuked and accused you.
"That you are full of goodness." This refers to the exhortation he has just given. The Apostle speaks as though saying: I did not regard you as harsh or as haters of your brethren when I advised you to receive one another, and not to abandon or destroy the work of God. I know that you are full of goodness.
By goodness, I think, he here means the fullness of virtue. He did not say, "you have goodness," but, "you are full of goodness." With the same expressiveness he continues: "filled with all knowledge."
What profit would there be if, with all their love, they did not know how to deal with those they loved? Therefore the Apostle added, "filled with all knowledge, able also to admonish others." They are able not only to learn, but also to teach others.