John Chrysostom
"By the power of signs and wonders, by the power of the Spirit of God": see Rom 15:18.
Then, in confirmation of this, he points to the multitude of disciples, since this too had been mentioned. Therefore he added: "so that from Jerusalem and round about as far as Illyricum I have fulfilled the Gospel of Christ."
Thus he enumerates cities and countries, peoples and tribes, not only within the Roman dominion, but also among the barbarians. Do not only complete the journey through Phoenicia, Syria, Cilicia, and Cappadocia. Imagine also all the peoples lying beyond them: the Saracens, Persians, Armenians, and the other barbarians.
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The Apostle said "round about" so that you would not go only by the straight and main road, but by every road, and would mentally penetrate even into South Asia.
Just as, by saying "in the power of signs and wonders," he expressed in one word the whole multitude of miracles, so also in the one word "round about" he again gathered countless cities, peoples, tribes, and countries.
In general, he was far from every form of boastfulness. He said this for the Romans, so that they would not think much of themselves. At the beginning he said, "that I might have some fruit among you also, as among the other Gentiles" (Rom 1:13). Here he appeals to the obligation of the priesthood.
Since earlier he had used rather severe expressions, here he shows his authority more clearly. There he simply said, "as among the other Gentiles." Here he also points to all the places of his preaching. In this way he undermines their arrogance from every side.
He did not simply say, "to preach the Gospel," but "to fulfill the Gospel of Christ."