I don't wish to underestimate the depth of this disagreement, but things have changed since the Early Church. At that time, when Jesus had come the 1st time, it was the beginning of a long age of punishment for Israel. It is often disputed, but the words of Jesus, I believe, are undisputable...
Luke 21.20 “When you see Jerusalem being surrounded by armies, you will know that its desolation is near. 21 Then let those who are in Judea flee to the mountains, let those in the city get out, and let those in the country not enter the city. 22 For this is the time of punishment in fulfillment of all that has been written. 23 How dreadful it will be in those days for pregnant women and nursing mothers! There will be great distress in the land and wrath against this people. 24 They will fall by the sword and will be taken as prisoners to all the nations. Jerusalem will be trampled on by the Gentiles until the times of the Gentiles are fulfilled....
32 “Truly I tell you, this generation will certainly not pass away until all these things have happened. 33 Heaven and earth will pass away, but my words will never pass away.
Sadly, these words have been a cause of endless debate throughout many centuries. But times have changed. When Jesus 1st gave this prophecy, Israel was entering into a period of judgment, while at the same time a new age would begin for Gentile Christians in the area of the Roman Empire.
The nation Israel, with the exception of a few Christians, rebelled against Christ and lost their temple religion, as well as their homeland. And they entered into the Jewish Diaspora, to last in effect until the end of the age.
Actually, the present age is biblically defined by this period of Jewish punishment. The end of this age is defined, by Jesus, as the end of Jewish punishment and the salvation of the nation Israel.
The salvation of "all Israel" refers not to the salvation of every individual in Israel, but rather, to the salvation of an entire nation, as opposed to the deliverance of just a remnant. This is a political salvation, as well as a Christian salvation. The nation will turn to Christ at his coming, and Israel, as a nation, will never again fall under national punishment.
This is often confused as a claim that every individual in the nation will get saved, spiritually. This is *not* what the words mean! This is a well-worn concept in the Prophets that Israel goes through changes, from one generation to another, falling away and then returning, coming under punishment, and then being restored. At times the nation was just partially delivered. But this promise involves the hope of a full national deliverance, to last for all time.
In the early Church Christians, after a few centuries, began to give up hope in this promised restoration of Israel. Jesus' statements, and Paul's statements on this were either ignored or reinterpreted, giving rise to "Replacement Theology." Of course, it wasn't called that, because "Israel" came to be viewed as the new name for the predominantly Gentile Church. Jewish Christians were not cast out of the Church, but the idea of a full nation being saved was discarded because of Israel's experience and failure.
But throughout Christian history it was recognized that full nations had become "Christian," if only in name. In reality, though, they were "Christian" philosophically and religiously, regardless of how many imposters or failures there were. These nations adopted Christianity as their state religion. All of the countries of Europe have, at one time or another, become "Christian States." They are now turning away from Christianity and engaging in religious apostasy, but they have all, at one time, converted, as nations, to being Christian States or Nations.
So the idea of "all Israel" becoming a Christian nation cannot have been dismissed except for the fact the Jews had lost their nation and were not turning in sufficient numbers to form a Christian nation. However, in modern times, we're seeing the possibility not just of a Christian nation of Israel, but also the viability of the Israeli nation. This didn't require sufficient numbers of Christian Jews to accomplish this, but it is now thought that like Ezekiel said Israel will be restored not by any virtue that they possess, but only for the sake of God's honor and name.
Eze 36.32 I want you to know that I am not doing this for your sake, declares the Sovereign Lord. Be ashamed and disgraced for your conduct, people of Israel!