Note 2 at Ro 15:9: Paul was briefly verifying a point that he had already made in this letter to the Romans. In Ro 15:9-12, he quoted four Old Testament scriptures in order to verify that Christ opened up the door of salvation to the Gentiles (Ps 18:49, De 32:43, Ps 117:1, and Isa 11:10; see also 2Sa 22:50 and Mt 12:21).

This was done to make it clear that Gentiles do not have to become Jews in order to be saved. The salvation of Gentiles as Gentiles does not fall into the category of one of those nonessential doctrines (see note 3 at Ro 14:1) discussed in Ro 14, on which we compromise for the sake of our weak brethren.