Romans 11:25-27 (NKJ)

The Salvation of “All Israel”

25 “For I do not desire, brethren, that you should be ignorant of this ministry, lest you should be wise in your own opinion, that blindness in part has happened to Israel until the fullness of the Gentiles has
come in.

26 And so all Israel will be saved, as it is written: The Deliverer will come out of Zion, And He will turn away ungodliness from Jacob;

27 For this is My covenant with them, When I take away their sins”

(See notes on 11:25 and 11:26 in the NKJ Version or below)

Note on v. 11:25
Israel's rejection is temporary, until all those who are going to be saved from among the Gentiles come to trust in Christ. Then salvation will come to a large number of Jews in the same way it has come to people for centuries - a faith response to the preached gospel of Christ.

Note on v. 11:26
And so: That is, "in this way, in the process just described." All Israel does not mean that every Jewish person who has ever lived will be saved, for Paul does not teach that (see 10:2,3)*
The term must be understood in the same sense as "the fullness of the Gentiles" (v. 25) Paul is therefore speaking in a collective sense. There will be a mighty turning on the part of the Jews to Christ.
Although many scholars believe the NT does not specify the time of Israel's ingathering (see note on v. 12), others see here a possible reference in Paul's statement, the Deliverer will come. Those taking this latter view see this quote of Isaiah 59:20 as a reference to Christ's return. Hence, many Jews, having seen and heard the truth of the gospel during the Great Tribulation, will accept the Messiah upon His return.

*Romans 10:2,3
2 For I bear them witness that they have zeal for God, but not according to knowledge.
3 For they being ignorant of God's righteousness, and seeking to establish their own righteousness, have not submitted to the righteousness of God.

Note on v. 11:12
Paul begins a theme that will return in vv. 15, 23, 24, 25 and 26: God has planned that in the future there will be a massive ingathering of Jewish people as they come to belief in Christ and are reconciled to God. This future revival of faith among the Jewish people is here called their fullness, "their acceptance" (v. 15) and their being "grafted into their own olive tree" (vv. 23, 24). Many scholars believe the NT does not specify the exact time of this ingathering. See note on v. 26

Isaiah 59:20
"The Redeemer will come to Zion,
And those who turn from transgression in Jacob,"
Says the Lord.