Rom 16:26-27, Acts 13:47-48, Mt 28:18-19, Mk 16:15, Acts 1:8

In the Old Testament, it was prophesied that all peoples of the nations would return to God. (Psalm 22:27-31)

Psalm 22:27 All the ends of the world shall remember and turn unto the Lord: and all the kindreds of the nations shall worship before thee. 28 For the kingdom is the Lord's: and he is the governor among the nations. 29 All they that be fat upon earth shall eat and worship: all they that go down to the dust shall bow before him: and none can keep alive his own soul. 30 A seed shall serve him; it shall be accounted to the Lord for a generation. 31 They shall come, and shall declare his righteousness unto a people that shall be born, that he hath done this.

As prophesied through the prophets in the Old Testament, Gentiles believed in Jesus as the Christ and obeyed God. (Romans 16:26-27)

Romans 16:26 But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith: 27 To God only wise, be glory through Jesus Christ for ever. Amen.

The Old Testament prophecy of the salvation of the Gentiles through Christ was fulfilled. In other words, the Gentiles believed in Jesus as the Christ and were saved. (Acts 13:47-48)

Acts 13:47 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.

Jesus commands us to go to all nations and tell them that Jesus is the Christ. (Mt 28:18-19, Mark 16:15, Acts 1:8)

Mt 28:18 And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. 19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:

Mark 16:15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.

Acts 1:8 But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.