Albert Barnes 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. Isaiah 52:15. This is not literally quoted but the sense is retained. The purpose of quoting it is to justify the principle on which the apostle acted. It was revealed that the gospel should be preached to the Gentiles; and he regarded it as a high honor to be the instrument of carrying this prediction into effect.