When the Jews rejected Jesus, what did Jesus do? Note what He said in Matt 23:37 - 39:
"O, Jerusalem, Jerusalem," - He was talking to the Jews - "the one who kills the prophets and stones those who are sent to her! How often I wanted to gather your children together, as a hen gathers her chicks under her wings, but you were not willing!
"See! Your house is left to you desolate;
"for I say to you, you shall see me no more" - note He did not say you shall see Me no more forever - "you shall see me no more till" - there's an 'until'! - "till you say, 'Blessed is he who comes in the name of the Lord!'"
Jesus was saying that the Jews would see Him no more until they were prepared to welcome Him.
After the Jews rejected Jesus, God opened the gates of salvation to the Gentiles (Rom 11:11). The good news is that after the fulness of the Gentiles is come to faith, then God will turn again to His Jewish people:
For I do not desire, brethren, that you should be ignorant of this mystery, lest you should be wise in your own opinion, that hardening in part has happened to Israel until the fullness of the Gentiles has come in.
And so all Israel will be saved. Rom 11:25, 26 (but read the rest of the chapter).
Israel is going to become a believing nation. When God turns again to the Jewish people, they will be ready to welcome Him. The Jews must come to faith, because of God's promises. God cannot forget His promises.