Isa 11:9-10, Rom 3:26-29, Rom 10:9-12
In the Old Testament, Solomon wanted Gentiles to come to Solomon's temple to pray to God. (1 Kings 8:41-43)
1 Kings 8:41 Moreover concerning a stranger, that is not of thy people Israel, but cometh out of a far country for thy name's sake; 42 (For they shall hear of thy great name, and of thy strong hand, and of thy stretched out arm;) when he shall come and pray toward this house; 43 Hear thou in heaven thy dwelling place, and do according to all that the stranger calleth to thee for: that all people of the earth may know thy name, to fear thee, as do thy people Israel; and that they may know that this house, which I have builded, is called by thy name.
In the Old Testament, it was foretold that the nations would return to God. (Isa 11:9-10)
Isa 11:9 They shall not hurt nor destroy in all my holy mountain: for the earth shall be full of the knowledge of the Lord, as the waters cover the sea. 10 And in that day there shall be a root of Jesse, which shall stand for an ensign of the people; to it shall the Gentiles seek: and his rest shall be glorious.
All who believe in Jesus Christ are justified and become children of God. (Romans 3:26-29, Romans 10:9-12)
Romans 3:26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus. 27 Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith. 28 Therefore we conclude that a man is justified by faith without the deeds of the law. 29 Is he the God of the Jews only? is he not also of the Gentiles? Yes, of the Gentiles also:
Romans 10:9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. 10 For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. 11 For the scripture saith, Whosoever believeth on him shall not be ashamed. 12 For there is no difference between the Jew and the Greek: for the same Lord over all is rich unto all that call upon him.