Romans 15:10

‘And again he says, “Rejoice, you Gentiles, with his people.” '

‘And again he says.' Here the Scriptures are seen as supplying the voice of God (‘He says'). This citation is taken from Deuteronomy 32:43. While there is no Messianic connection there it advances the previous theme of the Gentiles glorifying the God of Israel, while including the extra thought that they will do so along with God's own people. The two are to be united as one in their praise of God, as indeed they were in the church in Rome. That is why it was important that Jewish and Gentile Christians showed consideration for each other as described in chapter 14.