Adam sins and brings the death penalty upon all mankind.
In the beginning all people were the same. The Lord chooses Abraham and his descendants, the Israelites, to use in His plan of salvation to save all mankind.
The Lord makes a covenant with Israel which would make them a peculiar treasure to Him above all people.
This covenant, now often called the Old Covenant or the Law, separated the Israelites from the rest of mankind. Now you have two groups of people: the Israelites and the Gentiles.
The Lord’s plan is to save all mankind, not just Israel, which includes the Jews. In order for the Lord to save everyone He will have to make the two groups of people back into one people. The Lord does this because if He comes to the Jews and they accept Him then only the Jews would have access to eternal life. In order to make the Jews and the Gentiles one people again, He does away with the Old Covenant by the crucifixion of Jesus. Now there is neither Jew nor Gentile. The people are now one people again and salvation is offered to all mankind.
Ephesians 2:12-19: “That at that time ye were without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise, having no hope, and without God in the world: But now in Christ Jesus ye who sometimes were far off are made nigh by the blood of Christ. For he is our peace, who hath made both one, and hath broken down the middle wall of partition between us; Having abolished in his flesh the enmity, even the law of commandments contained in ordinances; for to make in himself of twain one new man, so making peace; And that he might reconcile both unto God in one body by the cross, having slain the enmity thereby: And came and preached peace to you which were afar off, and to them that were nigh. For through him we both have access by one Spirit unto the Father. Now therefore ye are no more strangers and foreigners, but fellow citizens with the saints, and of the household of God;”