John Calvin Commentary Romans 15:20

"yea, making it my aim so to preach the gospel, not where Christ was [already] named, that I might not build upon another man`s foundation;" — Romans 15:20 (ASV)

Thus striving to preach the gospel, etc. As it was necessary for Paul not only to prove himself to be the servant of Christ and a pastor of the Christian Church, but also to show his credentials for the status and office of an Apostle, so that he might gain the attention of the Romans, he mentions here the proper and peculiar distinction of the apostleship. For the work of an Apostle is to propagate the gospel where it had not been preached, according to that command:

“Go ye, preach the gospel to every creature.” (Mark 16:15)

And this is what we should carefully notice, so that we do not make a general rule of what specially belongs to the Apostolic order; nor should we consider it a fault that a successor was substituted who built up the Church. The Apostles then were the founders as it were of the Church; the pastors who succeeded them had to strengthen and amplify the building raised up by them. He calls that another’s foundation, which had been laid by the hand of another; otherwise, Christ is the only stone on which the Church is founded. See 1 Corinthians 3:11; and Ephesians 2:20.