Romans 15:21

But as it is written, To whom he was not spoken of, they shall see: and they that have not heard shall understand.

Cross-reference

Isaiah 52:15 is the exact source Paul quotes—those who have not heard will see. Direct citation.

Isaiah 66:19 describes survivors declaring God's glory to nations who have not heard. Mirrors the same unreached peoples theme.

Isaiah 65:1 speaks of God being found by those who did not seek him—a parallel theme of Gentiles receiving revelation.