In working on a related question I realized that there it was unclear to me who was the subject and who was the object in this verse:
Rom 11:15 KJV - 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?
GNT Romans 11:15 εἰ γὰρ ἡ ἀποβολὴ αὐτῶν καταλλαγὴ κόσμου τίς ἡ πρόσλημψις εἰ μὴ ζωὴ ἐκ νεκρῶν
The words translated "the casting away of them" seem like they could be read in at least two ways:
"For if their refusal/loss serves to bring about the reconciling of the KOSMOS..."
OR,
"For if by their being rejected reconciliation came to the KOSMOS..."
The same ambiguity is seen in the second clause:
The words translated "the casting away of them" seem like they could be read in at least two ways:
"...what will it mean if they are later received other than life from the dead?"
OR,
"...what will it mean if they later receive other than life from the dead?"
Since these are noun forms "refusal" and "reception" we don't have the benefit of the inflection of the verbs.
Possibly related:
Psa 94:14 KJV - 14 For the LORD will not cast off his people, neither will he forsake his inheritance.
I've always read it as the Jews being "cast off" but perhaps the point is that the LORD would not cut off his people but they were quick to cast off God and his Messiah.
Alternatively, might he be speaking of the "loss" of these unholy branches. That is how the same word is used here:
Act 27:22 KJV - 22 And now I exhort you to be of good cheer: for there shall be no loss[G580] of any man's life among you, but of the ship.
Was Paul saying "Their loss is our gain"? IE: "If their loss serves to bring about the reconciliation of the world, won't their reception be as much of a boon as life from the dead (which it is in fact)"? IE: Two boons from one loss? The boon to the gentiles because the non-elect Jews and the repentant non-elect Jews (when the hardening is lifted) and they "turn to the Lord":
Luke 15:24 KJV - 24 For this my son was dead, and is alive again; he was lost, and is found. And they began to be merry.
2 Corinthians 3:14-16 NLT - 14 But the people's minds were hardened, and to this day whenever the old covenant is being read, the same veil covers their minds so they cannot understand the truth. And this veil can be removed only by believing in Christ. 15 Yes, even today when they read Moses' writings, their hearts are covered with that veil, and they do not understand. 16 But whenever someone turns to the Lord, the veil is taken away.
Update
I'm adding the relevant section from a commentary. It examines the issue and takes a position. I'm posting it as part of the question to facilitate exposition, regardless of the position reached:
- if their rejection has meant. After vv 13–14, which were a sort of parenthetical remark, Paul turns now to repeat in different language what he said in v 12.
Some commentators (e.g., Black, Romans, 155; Cranfield, Romans, 562; Murray, Romans, 2. 81; Nygren, Romans, 397; Wilckens, Römer, 245) understand apobolē autōn, “their rejection,” as an objective gen., God’s (temporary) “rejection of them,” even comparing the gloss in Sir 10:20: “Fear of the Lord is the beginning of acceptance, but the beginning of rejection is obstinacy and arrogance.” But it is better taken as a subjective gen., i.e., the Jews’ rejection (of the gospel), in view of what Paul has exclaimed in 11:1, where he rejects the idea that God has rejected his own people. To introduce the idea of a temporary rejection of Israel by God is to read something into the text that is not there; it is nonetheless a very common interpretation of this phrase.
the reconciliation of the world. Although Paul does not explain this phrase here, he has already used a similar expression in 2 Cor 5:19, “God was in Christ, reconciling the world to himself,” i.e., setting it at one with himself. See Introduction, section IX.B. The providential aspect of Israel’s “rejection” has been the extension of reconciliation with God to all other human beings, and even a cosmic extension of that effect to the whole universe. Such a reconciliation will have the effect of making the ethnic Israel jealous and thus of drawing it closer to God’s new mode of salvation.
what will their acceptance mean? I.e., their acceptance or welcoming of the gospel, as the counterpart of their “rejection” of it (v 15a). Many commentators, however, think that “acceptance” means “acceptance by God,” appealing to 14:3 and 15:7; so Black (Romans, 155), BAGD, 717.
Fitzmyer, J. A., S. J. (2008). Romans: a new translation with introduction and commentary (Vol. 33, pp. 612–613). New Haven; London: Yale University Press.