23 And they also, if they do not continue in unbelief, will be grafted in, for God is able to graft them in again.
God has a promise for the nation Israel if she does not continue in unbelief (vv. 23-24). One day the nation Israel will meet the condition of belief in Jesus the Messiah. Thus, God has not permanently rejected Jews from salvation.
23 And they [the nation Israel who comes to believe in the Messiah] also,
There is a converse argument to verse 22 regarding the nation Israel. Verses 23 and 24 give the conclusion to the metaphor of the olive tree. Grace needs only faith to become operable. Once Israel as a nation places her faith in the Messiah, God will restore her to her national prerogatives.
if they [Israel] do not continue [persist] in unbelief,
God will extend justification to Israel as a nation if they believe in the grace of the Messiah. The one condition for Israel to enter into the Millennial kingdom is faith in Jesus the Messiah, that He was the atoning sacrifice for their sins.
will be grafted in,
God will graft the nation Israel back into the promise of the Abrahamic Covenant when she, as a nation, embraces Jesus as the Messiah by faith.
for God is able to graft them [Israel as a nation] in again.
The unbelief that caused Israel to be rejected by God can be reversed if she accepts by faith God’s provision. God is “able” because of His grace to graft those who believe into His covenant again by faith. Again, God has not abandoned Israel in His plan. It is easier to graft a natural branch (Israel) broken off from the original tree back into the olive tree than it is to graft in a wild or alien olive branch (Gentiles).
“Again” is a key to understand this verse. God will restore the nation Israel again. If Israel repents of her rejection of Jesus the Messiah, God will save her in grace (Jer 23:3-8).
Zech 12: 10 “And I will pour on the house of David and on the inhabitants of Jerusalem the Spirit of grace and supplication; then they will look on Me whom they pierced. Yes, they will mourn for Him as one mourns for his only son, and grieve for Him as one grieves for a firstborn.
If God accepted Gentiles without merit, He will restore Israel without merit.
God will one day reverse the destiny of the nation Israel. This grafting in again will happen at the end of the Great Tribulation. During this time great numbers of Jews will finally believe in Jesus the Messiah. They will receive the fulfillment of the unconditional promises of the Old Testament.
Ro 11: 26 And so all Israel will be saved, as it is written: “The Deliverer will come out of Zion, And He will turn away ungodliness from Jacob; 27 For this is My covenant with them, When I take away their sins.”
Belief by the Gentiles is what caused God to graft them in (v. 17), and Israel’s belief in the Messiah is what will cause God to reinstate that nation.
Ga 3: 14 that the blessing of Abraham might come upon the Gentiles in Christ Jesus, that we might receive the promise of the Spirit through faith.