Note 2 at Rom. 15:9: Paul briefly verifies a point that he has already made in this book to the Romans. He quotes four Old Testament scriptures (see above) to verify that Christ opened up the door of salvation to the Gentiles.

This is done to make it clear that Gentiles do not have to become Jews to be saved. Salvation to Gentiles as Gentiles does not fall into the category of one of those "non-essential" doctrines (see note 3 at Rom. 14:1, p. 838) that Paul discussed in chapter 14, on which we compromise for the sake of our weak brother.