It's because there has always been a faithful Jewish remnant(before and after Yeshua) that the nation of Israel has always existed.
Yeshua was very clear on what He expected of His people whenever the crowds would ask him how it was they could end up as a part of the Lord's own - and He made sure no one didn't know what he was about so that others would be uncertain:
Repent or Perish
Luke 13:21
13 Now there were some present at that time who told Jesus about the Galileans whose blood Pilate had mixed with their sacrifices. 2 Jesus answered, Do you think that these Galileans were worse sinners than all the other Galileans because they suffered this way? 3 I tell you, no! But unless you repent, you too will all perish. 4 Or those eighteen who died when the tower in Siloam fell on themdo you think they were more guilty than all the others living in Jerusalem? 5 I tell you, no! But unless you repent, you too will all perish.6............
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22 Then Jesus went through the towns and villages, teaching as he made his way to Jerusalem. 23 Someone asked him, Lord, are only a few people going to be saved?
He said to them, 24 Make every effort to enter through the narrow door, because many, I tell you, will try to enter and will not be able to. 25 Once the owner of the house gets up and closes the door, you will stand outside knocking and pleading, Sir, open the door for us.
But he will answer, I dont know you or where you come from.
26 Then you will say, We ate and drank with you, and you taught in our streets.
27 But he will reply, I dont know you or where you come from. Away from me, all you evildoers!
28 There will be weeping there, and gnashing of teeth, when you see Abraham, Isaac and Jacob and all the prophets in the kingdom of God, but you yourselves thrown out. 29 People will come from east and west and north and south, and will take their places at the feast in the kingdom of God. 30 Indeed there are those who are last who will be first, and first who will be last.
Paul himself, who wrote Romans 11 and "All Israel will be saved" had a heart to ensure that ALL sought to make themselves a part of the Remnant. For as He noted repeatedly, those Jewish believers who rejected Messiah did not consider themselves worthy of Eternal Life
Acts 13:47
When the Jews saw the crowds, they were filled with jealousy. They began to contradict what Paul was saying and heaped abuse on him.46 Then Paul and Barnabas answered them boldly: We had to speak the word of God to you first. Since you reject it and do not consider yourselves worthy of eternal life, we now turn to the Gentiles. 47 For this is what the Lord has commanded us:
I have made you[a] a light for the Gentiles,
that you[b] may bring salvation to the ends of the earth.[c]
According to Paul, despite the religious zeal of the Jewish people, they failed to understand the gift of Gods righteousness and therefore his hearts desire and prayer to God for [them was] that they may be saved (Romans 10:1).
Even Jewish people who are zealous for God (Romans 10:2) and are pursuing a law of righteousness (9:31; 10:3) are in need of salvation through Yeshua.
As for the notion that Jewish people can be saved by observing the Mosaic covenant, Paul wrote:
Now we know that whatever the law says, it says to those who are under the law, so that every mouth may be silenced and the whole world held accountable to God. Therefore no one will be declared righteous in his sight by observing the law; rather, through the law we become conscious of sin (Romans 3:19-20).
I do not set aside the grace of God, for if righteousness could be gained through the law, Christ died for nothing! (Galatians 2:20)
All who rely on observing the law are under a curse, for it is written: Cursed is everyone who does not continue to do everything written in the Book of the Law. Clearly no one is justified before God by the law, because, The righteous will live by faith. (Galatians 3:10-11)
That is why, to the end of his life, Paul reached out to his people: He longed to see them saved. And that is why he was willing to suffer so much persecution from his own people, coming back again and again to share the Good News (see, e.g., 2 Corinthians 11:24; Acts 21-22).
It is also important to remember that, in Jesus, God made a new covenant with the house of Israel and the house of Judah (see Jeremiah 31:31-34; Luke 22:19-20; Hebrews 8:7-12), and, By calling this covenant new, he has made the first one obsolete; and what is obsolete and aging will soon disappear (Hebrews 8:13). So, Israels way to God is through the new covenant rather than the Mosaic covenant, a point made emphatically clear with the destruction of the Temple in A.D. 70, a destruction that has lasted to this day.
Jesus made it clear that He was the fulfillment of the Torah and Prophets (see Matthew 5:17-19), while the disciples recognized Him to be the one of whom Moses and the prophets spoke (see John 1:45; Acts 3:24-26). After His resurrection, the Lord said to His disciples, This is what I told you while I was still with you: Everything must be fulfilled that is written about me in the Law of Moses, the Prophets and the Psalms (Luke 24:44), commissioning them to preach repentance and forgiveness of sins . . . in his name to all nations, beginning at Jerusalem (Luke 24:47).
Jesus told His fellow Jews that if they knew the Father, they would know Him also, and those who rejected Him rejected the Father as well (see Luke 10:16; John 5:36-47; cf. also 9:39-41). In keeping with this, John wrote that he who has the Son has life; he who does not have the Son of God does not have life, and that no one who denies the Son has the Father; whoever acknowledges the Son has the Father also (1 John 5:12; 2:23).
Paul had great sorrow and unceasing anguish in his heart when seeing how so many of his people were not saved (see Romans 9:2), including those whom he said were zealous for God, but their zeal is not based on knowledge (Romans 10:2). In fact, it was for those very people that he prayed (see Romans 10:1), Since they did not know the righteousness that comes from God and sought to establish their own, they did not submit to Gods righteousness (Romans 10:3).
IMHO, when understanding these necessary backgrounds/context - one can better understand the "ALL" of who Paul was talking about when it comes to Romans 11. ...and what it means to be a part of a Remnant.
IMHO, It is difficult to say which mystery Paul is talking about when he says this mystery in Romans 11. He could be talking about the olive tree and Gods kind and stern actions toward his people. Or this could refer to what he is about to say regarding Israels hardening but eventually acceptance. In some ways, I think it mostly points forward to what Paul is explains later and that's Gods plan to save Israel. That is, after all, what Paul is trying to explain all through chapters 9-11. The apostle Paul says God has planned the salvation of Israel in three steps 1) hardening of some of the Jews, 2) acceptance of some Gentiles, 3) all Israel will be saved. At this point we have some issues that need to be wrestled with. When Paul writes that all Israel will be saved we either have to define Israel by a certain set of people in order to make that consistent with all Paul has said about the necessity of righteousness to come by faith in Christ.... or we have to assume all Jews will eventually put their faith in Christ, or that God is going to save them no matter what (which is not consistent with what Paul has written to this point in Romans).
Some people have wanted to say Paul is talking about Spiritual Israel here but that just doesnt bear out through the context at all points...for it seems clear that Paul is talking about ethnic Jews here. Thus, either they all will eventually put their faith in Christ or Paul is defining Israel in a slightly different way than meaning every single person who is a direct descendent of Abraham.
It is beyond dispute that Paul and Jesus agree that not all ethnic Jews are actually children of Abraham (see John 8:39-41 & Romans 4:12).
Moreover, it has always been the case that God has called his people Israel when some Jews were not included in that number. Two places we see this in the OT are the concept of the remnant and also through blessings and curses in Deuteronomy..and for more on that, one can see the first paragraph in this post on Galatians 3:10-14. In Deuteronomy blessings and curses are repeatedly laid out before Gods people. The gist of it is, if you follow God and keep his commandments you will do well in the land. But if you double cross God and go your own way, you will be under a curse. This curse was basically considered a removal of the blessings of God and of covenant status with God.
Essentially, that basically would put an ethnic Jew out of Israel and into the same status as the Gentiles (who were also thought to be cursed and devoid of Gods blessings). Others may disagree, but my contention here when Paul says all Israel will be saved is that he is talking about true Israelites (as Jesus refered to Nathaniel as in John 1:47)
those who obeyed the law and were led by that law, as it was intended to do, to Jesus Christ. This does not mean every single Jew who ever lived will be saved and it does not mean that every single Jew will somehow get a second chance to believe in Jesus at the last day. It probably means faithful Jews will turn to Christ and be redeemed and that in some way, shape or form, there may be more of that to come than we realize.
Romans 11:26 could also mean that the physical Israeli State of today (or the future)/All Israelites who are NOT saved currently will later go through a great time of trial which will lead to much destruction--and yet those who remain will collectively turn to Yeshua together (just as it seems they collectively go AGAINST Yeshua currently aggressively in disbelief)...and when all the Jews left colletively repent, the Lord will save them---thus fulfilling the prophesy that "All Israel will be saved."
Who knows..
As said before elsewhere, there're others who've given some very dynamic thoughts on the subject. For more, one can consider
"The Naked Bible >> The Election of Israel and Its Meaning, Part 1" and
"The Naked Bible >> Romans 5:12 and Universalism: Applying My Take on Romans 5:12 to the Problem" (). Apart from that, some others that are beneficial, as it concerns the variety of quasi-universalism when it comes to the issue of "All Israel will saved" and none being able to not be called "children of the enemy":