Rom 4:16, Gal 3:16, Acts 2:38-39, Mt 28:19, Acts 1:8
In the Old Testament, God said that He would be the God of the Gentiles as well as the Israelites. (Deut 29:13-15)
Deut 29:13 That he may establish thee to day for a people unto himself, and that he may be unto thee a God, as he hath said unto thee, and as he hath sworn unto thy fathers, to Abraham, to Isaac, and to Jacob. 14 Neither with you only do I make this covenant and this oath; 15 But with him that standeth here with us this day before the Lord our God, and also with him that is not here with us this day:
Just as Abraham believed in the coming Christ, so the Gentiles who believe in Jesus are the heirs of God. (Galatians 3:16, Romans 4:16)
Galatians 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Romans 4:16 Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,
All who believe in Jesus as the Christ receive forgiveness of sins and receive the gift of the Holy Spirit. (Acts 2:38-39)
Acts 2:38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. 39 For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call.
So we must go to all the Gentiles and tell them that Jesus is the Christ so that they can be saved. (Matthew 28:19, Acts 1:8)
Matthew 28:19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
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.