Romans 15:20

St. Paul does not mean to say, that he never preached where the gospel had before been announced; this would not have been true, for he preached at Damascus, where there were already Christians, whom he formerly wished to take in chains to Jerusalem; and again in this epistle he announces the truths of the gospel to the Romans already converted by the preaching of St. Peter. But he means to say, that on these occasions he acts not as an apostle, whose office it is to preach to infidels; but as one that waters, confirms, comforts, as he says in the beginning of this epistle: and this he did as occasion offered, as the subsequent verses shew, where he tells us his design in calling on the Romans, in his journey to Spain. (Estius)