Rom 15:20

“"Yea, so have I strived to preach the Gospel, not where Christ was named." See here another preeminence; that he had not only preached the Gospel to so many, and persuaded them, but he did not even go to those who had become disciples. So far was he from thrusting himself upon other men's disciples, and from doing this for glory's sake, that he even made it a point to teach those who had not heard. For neither does he say where they were not persuaded, but "where Christ was not even named," which is more. And what was the reason why he had this ambition? "Lest I should build," he says, "upon another man's foundation." This he says to show himself a stranger to vanity, and to instruct them that it was not from any love of glory, or of honor from them, that he came to write, but as fulfilling his ministry, as perfecting his priestly duty, as loving their salvation. But he calls the foundation of the Apostles "another man's," not in regard to the quality of the person, or the nature of preaching, but in regard to the question of reward. For it was not that the preaching was that of another man, but so far as it went to another man's reward. For the reward of the labors of others was, to this man, another man's.”

“A man who builds on someone else's foundation is not doing anything wrong, as long as he builds with gold and such. Here Paul is referring to those false apostles who always went to converts and never to the Gentiles because they could not work miracles. Paul also shows that he had done a good job, for he both laid the foundation and also built on top of it.”

“I not only preached the gospel to these and so many peoples and converted them, but I also did not go to those people to whom the name of Christ had already been proclaimed. So far am I from subjecting other people's disciples to myself and doing this for my own glory. Therefore I wrote to you not out of a desire to gain glory from you, but because I am fulfilling my ministry. He calls the teaching of the apostles "another man's foundation" not because they were alien to him, nor because they preached something different, but in relation to the reward; because the reward for their labors was foreign to Paul, it did not belong to him.”

“Then when he says, and I have so preached, he shows how difficult it is to produce this fruit, for it is difficult to convert those entirely ignorant to the faith. First, therefore, he mentions the difficulty, saying: and I have so preached this Gospel, not, indeed, where Christ was named, i.e., not among those who had heard the name of Christ: people whom I had not known served me (Ps 18:43); behold, you shall call nations that you know not, and nations that knew you not shall run to you (Isa 55:5). Then he gives the reason, saying: lest I build on another man a foundation. Another man a foundation can be interpreted in two ways: in one way heretical doctrine, which is alien to the foundation of the true faith: in this sense the word, lest, is taken causally. For the Apostle willed to preach to those who had not heard the name of Christ with the intention, because if his hearers had heard the teachings of false apostles, it would have been more difficult to lead them back to the truth. Hence, it says in Matthew: the man is foolish who builds his house on sand (Matt 7:26), to which false doctrine is compared. In another way, another man a foundation can mean the doctrine of the true faith preached by others. Then the word, lest, is taken in a consecutive sense. For the Apostle did not avoid preaching to those to whom others had preached; thus he preached to the Romans, whom Peter had previously instructed. But when he preached to those who had heard nothing about Christ, the result was that he was not building on another's foundation, but he himself laid the first foundation of the faith: as a wise architect, I laid a foundation (1 Cor 3:10).”

“It was not without reason that Paul says that he tried to preach in places where Christ had not been named. For he knew that false apostles went about sharing Christ in ways which were wrong in order to ensnare the people by some other teaching under the name of Christ, which was then very difficult to put right afterward. Therefore he wanted to get there first, in order to preach the right message.”