Rom 11:25-27 "For I do not want you, brethren, to be uninformed of this mystery -- so that you will not be wise in your own estimation -- that a partial hardening has happened to Israel until the fullness of the Gentiles has come in; and so all Israel will be saved; just as it is written, "THE DELIVERER WILL COME FROM ZION, HE WILL REMOVE UNGODLINESS FROM JACOB. THIS IS MY COVENANT WITH THEM, WHEN I TAKE AWAY THEIR SINS."
The only way one can say that the Church is Israel in this passage is to suppose that the Church has been blinded, and that the Church is the enemy of the Gospel. This is, to put it bluntly, utterly ridiculous, not to mention heretical. Since Paul is speaking of national, racial Israel—the Israel of the flesh, if you wish—when he speaks of them being enemies to the Gospel, he is also speaking of national Israel when he says, "all Israel will be saved, as it is written," and quotes Isaiah. He is also, ergo, speaking of national Israel all through chapter 11.
Since Paul draws a distinction between the Church and Israel, it is obvious that one cannot simply say that the Church is "the New Israel," and speak of the replacement of the old. How then are we to understand their relationship? This is also covered in Romans 11, this time in verses 17-21: The Church is grafted into Israel's vine to replace the "natural branches" that were broken off for disbelief, "and if they do not persist in disbelief, they will be grafted in, for God is able to graft them in again" (v. 23).
It is not enough to simply say that those Jews who have come to Christ are the "all Israel" that is to be saved, for when Paul wrote Romans, many Jews had become Christians, and from the tone of the other epistles written, we know that they were still the driving force in the Church. Yet Paul does not consider this to be the fulfillment of God's promises to his people! It is clear that he is expecting the day when the whole nation of Israel will be reconciled to God.
One has to wonder how a whole nation, with no exceptions, could come to be saved. The answer is to be found throughout the Bible. Zechariah 12-14 goes into detail about the process: "And it shall come to pass, that in all the land, saith the LORD, two parts therein shall be cut off and die; but the third shall be left therein. And I will bring the third part through the fire, and will refine them as silver is refined, and will try them as gold is tried: they shall call on my name, and I will hear them: I will say, It is my people: and they shall say, The LORD is my God."
Yet even in all of Israel being saved, we see differences between them and the Church. When Christ comes for the Church, "the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord" (1 Thess. 4:16-17).
Yet when He comes for Israel, "his feet shall stand in that day upon the mount of Olives, which is before Jerusalem on the east, and the mount of Olives shall cleave in the midst thereof toward the east and toward the west, and there shall be a very great valley; and half of the mountain shall remove toward the north, and half of it toward the south. And ye shall flee to the valley of the mountains; for the valley of the mountains shall reach unto Azal: yea, ye shall flee, like as ye fled from before the earthquake in the days of Uzziah king of Judah: and the LORD my God shall come, and all the saints with thee" (Zech. 14:4-5).
The Church, including those Israelites who trusted Christ before He comes on the clouds, will be Resurrected and Raptured, risen into the clouds to be with Him. The nation of Israel, which will persist in unbelief until He comes and even those who pierced Him see Him, will remain mortal, but will be allowed to flee through the Mount of Olives to an unknown land to wait out Armageddon. The Church will be sheltered in the Lamb's tent, while 144,000 Israelites will need to be sealed to protect them from the Trumpets (Rev. 7:2-3, 15, 9:4). The Church is taken before the Day of the Lord, and Israel is told not to look forward to that Day (Amos 5:14).
So yes, there is a wide gap of difference in the Church, comprised of all nations, and Israel, a single nation set apart. We are grafted into Israel's branch for salvation, but we do not replace her. The Church has a separate destiny from Israel, but the day will come when "all Israel will be saved, as it is written." When that happens, all of the promises that God has made to Israel that were put on hold due to her disbelief will be fulfilled exactly as they were written.
You quote Romans 11:26: "And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:"
But obviously, that cannot mean literally that all Israel is going to be saved, because Paul had earlier said only a remnant will be saved. Rom 9:27 Esaias also crieth concerning Israel, Though the number of the children of Israel be as the sand of the sea, a remnant shall be saved:
Earlier in Romans, Paul said Jews and Gentiles, alike, were under condemnation. Rom 3:9 What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin; 10 As it is written, There is none righteous, no, not one: 11 There is none that understandeth, there is none that seeketh after God. 12 They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one. 13 Their throat is an open sepulchre; with their tongues they have used deceit; the poison of asps is under their lips: 14 Whose mouth is full of cursing and bitterness: 15 Their feet are swift to shed blood: 16 Destruction and misery are in their ways: 17 And the way of peace have they not known: 18 There is no fear of God before their eyes.
Then, in verses 23,24, he gives the only hope for Jews and Gentiles, alike: Rom 3:23 For all have sinned, and come short of the glory of God; 24 Being justified freely by his grace through the redemption that is in Christ Jesus:
Jews are not exempt from the requirement that they become Christians in order to be saved. And, as I already pointed out, only a few would do so. That's the remnant of Romans 9:27.
Again, to show that Jews hold no better place in God's eyes than Gentiles, Romans 9:6,7: Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel: Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
Again, Rom 2:28-29: For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.
Later, when Paul wrote Galatians (a truly excellent book, by the way), he said, "Know ye therefore that they which are of faith, the same are the children of Abraham." (Gal 3:7)
So all Israel here means all who believe in Jesus Christ, Jew or Gentile.
Gal 3:26 For ye are all the children of God by faith in Christ Jesus. 27 For as many of you as have been baptized into Christ have put on Christ. 28 There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. 29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.