Several answers might be offered to this question:
First, it could be proposed that “All Israel” refers to all physical descendants of Abraham that have lived since the time of the patriarchs. But then all the unbelieving Israelites across the ages would be saved without faith in Jesus as the Christ.
Secondly, it could be proposed that “All Israel” refers to all physical descendants of Abraham living at the time of the return of Christ. But is not that viewpoint extremely limiting in terms of a definition of “all Israel?” Are all Jews who have lived from the time of Abraham even until today excluded from being a part of “All Israel,” Abraham’s seed?
Thirdly, it could be proposed that “All Israel” refers to all physical descendants of Abraham and all people of other nationalities that have converted to Judaism across the ages. But then why not urge everyone to become Jews by being ritually circumcised if they cannot believe in Jesus as the Christ? Then they all would be sure to be saved if they were living when Christ returned? Can you imagine Paul’s reaction to that proposal? Would he not explain that keeping the law by being circumcised then obligates a person to keep the whole law perfectly to be saved?
Fourthly, it could be proposed that “All Israel” refers to all physical descendants of Abraham that have repented of their sins and believed in Jesus as the promised Christ. That certainly is a possibly correct answer to the question. But it clearly has a limiting factor in restricting the definition of “Israel” only to Jewish believers. Do not believers from all other nations have Abraham as their father? Are they not members of the Israel of God, and equal heirs to the promises? Is that not Paul’s point in Romans 4 and in Ephesians 2 and 3?
Fifthly, it could be proposed that “All Israel” refers all those who have repented of their sins and believed in Jesus as the promised Christ, whether they be physical descendants of Abraham or people of other nationalities. Yes, and this answer conforms most perfectly to the grand scheme of the history of redemption. Abraham was originally a Gentile, with ancestors worshipping idols on the other side of the river Euphrates. By the sovereign call of God and his response of faith, Abraham became the “father of many nations.” So he has two fatherhoods: (1) Abraham is the father of all believing Jews; and (2) Abraham is the father of all Gentiles who believe. These two groupings of Abraham’s children constitute the “Israel of God,” the “All Israel” of Romans 11:26.
How should this answer affect the Christian’s attitude toward the current struggle in the Middle East?
Christians all over the world should recognize that believing Jews and believing peoples of other nationalities are equally a part of the “Israel of God.” Together with believers from all the nations of the world, they constitute the “All Israel” of Romans 11:26. Together they are equally heirs of all the promises of God to Abraham, including the promise of the land, the seed and the blessing. For a fuller discussion of this critical question, see O. Palmer Robertson, The Israel of God. Yesterday, Today, and Tomorrow
(Phillipsburg: P & R Publishing, 2000). Note especially Chapter 6: “The Israel of God in Romans 11.”