When we read the Scriptures, we may be tempted in thinking that the world is divided up into two types of people - Jews and Gentiles. While that may be apparent in one way, the bible's distinction of the two types of people that really matter is much more significant than an ethnic qualification. In the Old Testament, to be a part of the people of God was to be Jewish. To be truly Jewish was not simply in the sense that your physical heritage was in Abraham but in the fact that you were called out in Abraham to have the same faith as Abraham. This is how non-Israelites could know salvation as they trusted in the same promise of the Offspring of blessing that was given to Abraham. There were believers in Old Testament Israel and non-believers in Old Testament Israel.
When we get to the New Testament, there is still much discussion about the distinction of Jew and Gentile, but the significance of the distinction is only seen in the fact that it is obliterated in Christ. I would argue that in both the Old and New Testament, the greatest distinction in humanity is where we stand with Jesus the Messiah. Perhaps it will be helpful to see this in a few different passages. Paul, the most zealous Jew who ever came to Christ makes these arguments in the most powerful ways.
In writing to the Galatian church, Paul was concerned that this predominantly gentile church was being influenced by a Jewish contingent to believe that they needed to adopt Jewish practices to be a part of God's people. Paul was concerned that the churches in Galatia were losing the gospel in their temptation to pick up the Jewish law. He made his point very clear in Galatians 2:15-16.
"We ourselves are Jews by birth and not Gentile sinners; 16 yet we know that a person is not justified by works of the law but through faith in Jesus Christ, so we also have believed in Christ Jesus, in order to be justified by faith in Christ and not by works of the law, because by works of the law no one will be justified." Faith in Christ is the only distinguishing factor for justification.
In his letter to the church in Ephesus, Paul makes a beautiful point that the warring hearts between Jew and Gentile is completely solved in Jesus. He does so by showing that there are never two distinct groups as God's people but only one people of God, one nation, one citizenship, one promise, one temple, one body, all in Christ Jesus.
Ephesians 2:11-21 Therefore remember that at one time you Gentiles in the flesh, called "the uncircumcision" by what is called the circumcision, which is made in the flesh by hands-- 12 remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world. 13 But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. 14 For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility 15 by abolishing the law of commandments expressed in ordinances, that he might create in himself one new man in place of the two, so making peace, 16 and might reconcile us both to God in one body through the cross, thereby killing the hostility. 17 And he came and preached peace to you who were far off and peace to those who were near. 18 For through him we both have access in one Spirit to the Father. 19 So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, 20 built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, 21 in whom the whole structure, being joined together, grows into a holy temple in the Lord.
For one more example among many, Paul explains to the predominantly gentile Roman church that there is one tree that signifies the people of God. While its roots are Jewish, the only way to be a part of that tree is to be in Christ.
Romans 11:16-21 If the dough offered as firstfruits is holy, so is the whole lump, and if the root is holy, so are the branches. 17 But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree, 18 do not be arrogant toward the branches. If you are, remember it is not you who support the root, but the root that supports you. 19 Then you will say, "Branches were broken off so that I might be grafted in." 20 That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear. 21 For if God did not spare the natural branches, neither will he spare you.
If Jews who rejected Christ are broken off from the one tree it is only in the acceptance of Christ that both Jews and Gentiles can be grafted in. There is one tree and one people of God.
At the end of the day this tells us something very important. There are truly only two kinds of people in the world. Those who are in Christ and those who are not. It makes an eternal difference which group you are in and reminds us how much everyone, both Jew and Gentile, need to hear and believe the gospel.