Rom 9:27 Isaiah also cries out concerning Israel: "Though the number of the children of Israel be as the sand of the sea, the remnant will be saved.
In the OT, saving of the Jews, who were already the people of God, would be the physical salvation from warfare, destruction and annihilation.
But in the NT, the Jews had lost their salvation for out of their unbelief they crucified their Messiah. They also cursed themselves to damnation for shedding innocent blood of our Lord Jesus. (Read also U29 - great and dreadful day of the Lord)
Matthew 27:24-25 When Pilate saw that he could not prevail at all, but rather that a tumult was rising, he took water and washed his hands before the multitude, saying, "I am innocent of the blood of this just Person. You see to it." And all the people answered and said, "His blood be on us and on our children."
2 Tim 2:13 If we are faithless, He remains faithful; He cannot deny Himself.
Though the Israelites had been unfaithful to Him since the day they left Egypt, yet He is faithful to His promise. He gave them Judaism, but they became hypocrites.
Out of His great love and faithfulness to Abraham and Jacob, He came to earth to be the Lamb of God and fulfilled all the requirements of the Mosiac Law thereby making the law obsolete and setting the Jews free from the law (Judaism).
At His death on the cross He established the new covenant of His blood with the Jews (and also with Gentiles) so that whoever believes in Him will be saved.
During our Lord's 3-4 years on earth, He only managed to disciple 200 Jews for most of the Jews were hypocrites. Thousands followed Him mainly because of food and miracles. At Pentecost, the 200 Jews were filled with the Holy Spirit and the 3,000 devout Jews from afar were also saved. Later another 5,000 in Jerusalem were saved.
Rom 11:5-7 Even so then, at this present time there is a remnant according to the election of grace. And if by grace, then it is no longer of works; otherwise grace is no longer grace. But if it is of works, it is no longer grace; otherwise work is no longer work. What then? Israel has not obtained what it seeks; but the elect have obtained it, and the rest were blinded.
The Priesthood and Jewish leaders (representing Israel) then were hypocrites and blind and so they rejected the Messiah and salvation, but the elect will be saved by the grace of God.
The Devout Jews were the Elect
John 5:46 "For if you believed Moses, you would believe Me; for he wrote about Me."
The elect were devout Jews, who had kept the Law of Moses. By keeping His words, they were in the light and could believe and accept our Lord Jesus Christ.
Psalm 119:105 Your word is a lamp to my feet and a light to my path.
Remnants were saved
Eze 18:30-31 "Therefore I will judge you, O house of Israel, every one according to his ways," says the Lord God. " Repent, and turn from all your transgressions, so that iniquity will not be your ruin. Cast away from you all the transgressions which you have committed, and get yourselves a new heart and a new spirit. For why should you die, O house of Israel?
The iniquity of killing their Messiah condemned all Jews to their death (damnation). The new covenant provides salvation to all repented Jews.
Overnight, the unlearned fishermen filled with the new heart zealously proclaimed His love and with the new Spirit of truth in them, they fearlessly preached the gospel of Jesus to the Jews.