A.T. Robertson 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)

As it is written (καθως γεγραπτα). From Isa 52:15. Paul finds an illustration of his word about his own ambition in the words of Isaiah. Fritzsche actually argues that Paul understood Isaiah to be predicting his (Paul's) ministry! Some scholars have argued against the genuineness of verses 9-21 on wholly subjective and insufficient grounds.