John Calvin Commentary Romans 15:21

"but, as it is written, They shall see, to whom no tidings of him came, And they who have not heard shall understand." — Romans 15:21 (ASV)

But as it is written, etc. He confirms by the testimony of Isaiah what he had said about the evidence of his apostleship. For in Isaiah 52:15, speaking of the kingdom of the Messiah, among other things he predicts that the knowledge of Christ would be spread among the Gentiles throughout the whole world, and that his name would be declared to those who had not heard of it before. It was fitting that this should be done by the Apostles, to whom the command was specifically given. Therefore, Paul's apostleship was made evident by the fact that this prophecy was fulfilled in him.

It is absurd for anyone to attempt to apply what is said here to the pastoral office. For we know that in churches properly established, where the truth of the gospel has already been received, Christ’s name must be constantly preached. Paul, then, was a preacher of Christ who was as yet unknown to foreign nations, for this purpose: that after his departure, the same doctrine should be proclaimed daily in every place by the pastors. For it is certain that the Prophet speaks of the beginning of the kingdom of Christ.