[T]he most likely explanation of ‘all Israel’ is that it means the nation as a whole, though not necessarily including every individual member. [W]e understand ‘shall be saved’ to refer to a restoration of the nation of Israel to God at the end of history, an eschatological event in the strict sense… Some light on the meaning of this ‘shall be saved’ in Paul’s mind may be expected from the Old Testament quotation which follows.
Rom. 26b-27. as it is written: ‘Out of Zion shall come the Deliverer, he shall turn away iniquities from Jacob. And this is the covenant I will make with them, when I take away their sins.’ …The original reference of Isaiah 59:20 may refer to God Himself, but there is some Rabbinic evidence that it came to be interpreted of the Messiah, and it is likely that Paul so understood it. The coming he probably understood of the Parousia, and ‘Zion’ he probably interpreted as denoting heaven or the heavenly sanctuary. The words ‘he shall turn away iniquities from Jacob’ (here the Septuagint differs considerably from the Hebrew) indicate the nature of the deliverance which this Deliverer will accomplish; it will consist of turning back ungodliness from the nation of Israel. Such a characterization of the work of the Messiah affords a striking contrast to the Jewish expectation of a political messiah… The effect of the substitution of a clause from the Septuagint version of Isaiah 27:9 for the latter part of the verse brings out forcefully the fact that the essence of the new covenant which God will establish with Israel is His gracious forgiveness of their sins. The composite quotation thus makes clear the nature of the deliverance indicated by ‘shall be saved’ in v. 26a by its relentless concentration on God’s forgiveness and on Israel’s need of it. It dashes Israel’s self-centred hopes of establishing a claim upon God, of putting Him under an obligation by its merits, making clear that the nation’s final salvation will be a matter of the forgiveness of its sins by the sheer mercy of God. It is also to be noted that there is here no trace of encouragement for…anything which could feasibly be interpreted as a scriptural endorsement of the modern nation-state of Israel.
~ C.E.B. Cranfield, Romans: A Shorter Commentary