Before Jesus ascended into heaven, His disciples asked, "Lord, will you at this time restore the kingdom to Israel?" He said, "It is not for you to know times or seasons that the Father has fixed by His own authority. But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem, in Judea and Samaria, and to the ends of the earth" (Acts 1:6-8).
The disciples believed as most Jews did that the Messiah was going to make Israel great again, as it was under kings David and Solomon. But Jesus said that with the Holy Spirit, you will go into the world preaching the gospel, and this will be done for as long as the Father has determined.
In other words, Jesus was saying that the preaching of the gospel will Israel. Whoever believes the gospel of Jesus Christ is true Israel. Now some will decry this as supersessionism: "The church has not replaced Israel!" they will say. Of course it hasn't. The church is the expansion of Israel.
Paul referred to true believers as the Israel of God (Galatians 6:15-16). Romans 2:28-29 says, "No one is a Jew who is merely one outwardly, but a Jew is one inwardly, and circumcision a matter of the heart." Romans 9:6 says, "For not all who are descended from Israel," meaning ethnic Israel, "belong to Israel," meaning spiritual Israel.
In Romans 11, we're given the picture of a cultivated olive tree: branches have been cut off, and Jews and Gentiles grafted into Christ together. Then verse 26 says, "And in this way all Israel will be saved," meaning true Israel, all who serve Christ as king, when we understand the text.
"I tell you many will come from east and west and recline at table with Abraham, Isaac, and Jacob in the kingdom of heaven, while the sons of the kingdom will be throne into the outer darkness. In that place there will be weeping and gnashing of teeth." Matthew 8:11-12
"Bear fruits in keeping with repentance. And do not begin to say to yourselves, 'We have Abraham as our Father.' For I tell you God is able from these stones to raise up children for Abraham." Luke 3:8
"They answered Him, 'Abraham is our father!' Jesus said to them, 'If you were Abraham's children, you would be doing the works that Abraham did..." John 8:39
"Know then that it is those of faith who are the sons of Abraham... And if you are Christ's then you are Abraham's offspring, heirs according to promise." Galatians 3:7, 29