
The background starts in the beginning. Genesis 1-11 covers 2000 years in 11 chapters. Genesis 12-50 covers only 400 years in 39 chapters. Beginning in Gen 2, Adam and Eve were in innocence in the Garden of Eden. That age of innocence ended in failure with the fall into sin.
Next, in Gen 3-6, God dealt with the growing human race by leaving them to act according to conscience. But it got so bad that God wiped out the entire world except for Noah and started over with him and his family.
So starting in Gen 12 God does something different again. He takes one individual (Abraham) and makes him a promise based on Abraham’s faith. God begins to fulfill that promise miraculously with the birth of Isaac. So now God begins a process of redemption that starts with one family, to redeem the whole human race.
So when Christ arrives in the "fullness of time," at least five different ways of dispensing salvation have been tried (five “dispensations”) and each one ended in human failure. God’s answer is to take it upon himself to redeem mankind, and he does so when Christ comes. This begins the dispensation of grace.
Jesus Christ was the fulfillment of the promise of a Jewish Messiah. So he first had to go to the Jews to offer them the kingdom. The four gospels record how they rejected both the kingdom and the king, and crucified him. God simply turned that into part of his plan, and inaugurated a “mystery,” which he explains to Paul as “the church age.” In the dispensation of the grace of God, the kingdom of God is spiritual, and the blessings of the new covenant are given to Gentiles without them having to fulfill any Jewish obligations.
Paul explains this in Romans 9-11 as God’s way of making the Jews jealous in order to draw them back to Christ. This will be successful, but not until the end of the church age, after the church (which is the body and bride of Christ) is raptured to be with him before the Second Coming.