Rom 9:24-25, Rom 15:9-11
In the Old Testament, Isaiah prophesied that God would make the Gentiles His people in the future. (Isa 19:24-25)
Isa 19:24 In that day shall Israel be the third with Egypt and with Assyria, even a blessing in the midst of the land: 25 Whom the Lord of hosts shall bless, saying, Blessed be Egypt my people, and Assyria the work of my hands, and Israel mine inheritance.
Paul revealed that, according to the prophecies of the Old Testament, God made Gentiles also to believe in Jesus as the Christ and made them His people. (Acts 13:46-48, Romans 9:24-25, Romans 15:9-11)
Acts 13:46 Then Paul and Barnabas waxed bold, and said, It was necessary that the word of God should first have been spoken to you: but seeing ye put it from you, and judge yourselves unworthy of everlasting life, lo, we turn to the Gentiles. 47 For so hath the Lord commanded us, saying, I have set thee to be a light of the Gentiles, that thou shouldest be for salvation unto the ends of the earth. 48 And when the Gentiles heard this, they were glad, and glorified the word of the Lord: and as many as were ordained to eternal life believed.
Romans 9:24 Even us, whom he hath called, not of the Jews only, but also of the Gentiles? 25 As he saith also in Osee, I will call them my people, which were not my people; and her beloved, which was not beloved.
Romans 15:9 And that the Gentiles might glorify God for his mercy; as it is written, For this cause I will confess to thee among the Gentiles, and sing unto thy name. 10 And again he saith, Rejoice, ye Gentiles, with his people. 11 And again, Praise the Lord, all ye Gentiles; and laud him, all ye people.