Albert Barnes on Romans 15:21

But as it is written. Isa 52:15. This is not literally quoted but the sense is retained. The design 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 honour to be the instrument of carrying this prediction into effect.

(d) "To whom he was not spoken of" Isa 52:15