Herman Witsius, The Economy of the Covenants Between God and Man, Vol. 2, pp. 414-415:
XXII. From what we have said before, it appears, that they depart from the apostle's meaning, who, by all Israel, understand the mystical Israel, or the people of God, consisting both of Jews and Gentiles, without admitting the conversion of the whole Jewish nation to Christ, in the sense we have mentioned. Notwithstanding this may be confirmed by the following arguments, 1st. The apostle speaks of that Israel, to whom he ascribes his own pedigree, ver. 1 whom he calls his flesh, that is, his kindred, ver. 14. and the natural branches, ver. 21. whom he constantly distinguishes from the Gentiles; to whom, he testifies, blindness is happened. All this is applicable to Israel properly so called. 2dly. He lays before us a mystery: but it was no mystery that a very few Jews were converted to Christ together with the Gentiles; for we have daily instances of that. 3dly. He reminds the Gentiles not to exult over, or despise the Jews, from this argument, that, as they themselves were now taken in among the people of God, so, in like manner, the Jews were in due time to be taken in again. But if the apostle meant, that the body of the Jewish nation was to continue in their hardness; and but a few of them to be saved, who, joined to the Gentiles, should form a mystical Israel, the whole of that discourse would be more adapted to the commendation of the Gentiles than of the Israelites; and encourage rather than repress the pride of the Gentiles. 4thly. As the fall and diminishing of Israel, ver. 12. and their casting away, ver. 15. are to be understood; so likewise the receiving and saving of them; for here the rules of a just opposition must be observed. But the fall, diminishing, and casting away of Israel are to be understood of the generality of the Jewish nation; therefore the receiving and saving of Israel in like manner.
He sums up the expectation Christians ought to have based on this exegesis of Romans 11 (p. 418):
As from all this it is evident, we are to expect the general conversion of the Israelites in time to come, not indeed of every individual, but of the whole body of the nation, and of the twelve tribes.
And how is this promise of God a "benefit" for which Christians are to be thankful? Witsius answers (p. 419):
Lastly, To this restoration of Israel shall be joined the riches of the whole church, and as it were, life from the dead, Rom. xi. 12. "Now if the fall of them be the riches of the world, and the diminishing of them the riches of the Gentiles; how much more their fulness?" and ver. 15. "For if the casting away of them be the reconciling of the world; what shall the receiving of them be, but life from the dead?" The apostle intimates, that much greater and more extensive benefits shall redound to the Christian church from the fulness and restoration of the Jews, than did to the Gentiles, from their fall and diminuation; greater, I say, intensively, or with respect to degrees, and larger with respect to extent.
What a day to look forward to, and to hasten, if the Lord wills, by our humble prayers for the conversion of the Jews. As Samuel Rutherford memorably wrote, Letters, XXXIX, p. 89:
Oh, to see the sight next to Christ's coming in the clouds, the most joyful! our elder brethren, the Jews, and Christ fall upon one another's necks, and kiss each other! They have been long asunder, they will be kind to one another when they meet: O day! O longed for, and lovely day, dawn! O sweet Jesus, let me see that sight that will be as life from the dead, thee and thy ancient people in mutual embraces!