http://www.gotquestions.org/all-Israel-saved.html
". . .In this way all Israel will be saved. As it is written: “The deliverer will come from Zion; he will turn godlessness away from Jacob. And this is my covenant with them when I take away their sins.” As far as the gospel is concerned, they are enemies for your sake; but as far as election is concerned, they are loved on account of the patriarchs, for God’s gifts and his call are irrevocable.
Here, Paul emphasizes the “irrevocable” nature of Israel’s calling as a nation. Isaiah predicted that a “remnant” of Israel would one day “be called the Holy People, the Redeemed of the LORD” (Isaiah 62:12). Paul speaks of the “full inclusion” of Israel in the future (Romans 11:12). Regardless of Israel’s current state of unbelief, a future remnant will in fact repent and fulfill their calling to establish righteousness by faith (Romans 10:1–8; 11:5). This conversion will fulfill Moses’ prediction of Israel’s permanent restoration to the land (Deuteronomy 30:1–10). God’s larger redemptive plan involves both Jews and Gentiles. When Paul says Israel will be “saved,” he means their deliverance to this physical inheritance as integral to God’s ultimate plan (Romans 11:30–36).
So how will “all Israel be saved” and restored in the land? The details of this deliverance are filled out in p***ages such as Zechariah 8—14 and Revelation 7—19, which speak of end-times Israel at Christ’s return. The key verse describing the coming to faith of the future remnant of Israel is Zechariah 12:10, “I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and supplication. They will look on me, the one they have pierced, and they will mourn for him as one mourns for an only child, and grieve bitterly for him as one grieves for a firstborn son.” This occurs during the tribulation prophesied in Daniel 9:24–27. The apostle John references this event in Revelation 1:7. The faithful remnant of Israel is epitomized in Revelation 7:1–8 and 11:1–12. These faithful ones the Lord will save and bring back to Jerusalem “in truth and righteousness” (Zechariah 8:7–8, NASB).
The tribulation period will feature unprecedented apostasy in Israel for 3½ years, with a “second exodus” arranged by God to protect the faithful remnant from Satan (Revelation 11—12) just as in the first Exodus. Isaiah predicted this as well: “In that day the Lord will reach out his hand a second time to reclaim the surviving remnant of his people” (Isaiah 11:11). The gospel will be preached to all the world, and Christ will return to meet the faithful remnant (Revelation 14) and destroy the armies gathered against Him in rebellion (Revelation 19). The apostates left in Jerusalem will be purged, and the remnant set apart forever as God’s holy people (Zechariah 13:8—14:21). Isaiah 12 is their song of deliverance: Zion will rule over all the nations defeated under the banner of Messiah the King, and Israel’s “salvation” is the wholeness and peace she will enjoy during the millennial reign of Christ. . ."