25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fullness of the Gentiles be come in.
Greeks at the time had religions which restricted membership and did not disclose their “mysteries” till you got inside. Paul uses that concept here and other places to demonstrate to the Gentiles that God had a plan that He is now fulfilling. The Israelites didn’t know ahead of time that salvation would apply to Gentiles. Now everyone was realizing that the Gentiles could be saved.
The fullness of the Gentiles refers to the spread of the Gospel thought out the world. The partial hardening implies that not all Jews are hardened, but there is a remnant left. The “until” hints that the end of the hardening is upcoming.
26 And so all Israel shall be saved: as it is written, There shall come out of Zion the Deliverer, and shall turn away ungodliness from Jacob: 27 for this is my covenant unto them, when I shall take away their sins.
Israel as a nation will be recovered to God. Paul bases this on the covenant God made with Abraham. The three R’s show us that Paul is talking about physically Israel and not believers. Paul has been talking about a remnant of physical Israel, the replacement of physical Israel by the Gentiles and the restoration of physical Israel in the end. Israel will once again become Israel.
In the future, when God is restoring all things, both the national and spiritual blessings promised to Abraham will come together. The national Jews will be spiritual Jews, as the Jews turn away from their sins and back to God and are made righteous through the promise to Abraham.
Paul quotes from Isaiah 59:20:
59:20 And a Redeemer will come to Zion, and unto them that turn from transgression in Jacob, saith Jehovah. 21 And as for me, this is my covenant with them, saith Jehovah: my Spirit that is upon thee, and my words which I have put in thy mouth, shall not depart out of thy mouth, nor out of the mouth of thy seed, nor out of the mouth of thy seed's seed, saith Jehovah, from henceforth and for ever.
The context of which is Israel sins and God’s coming judgment. The Lord is depicted as a warrior coming to clean house. After that He will come as a redeemer to save. Isaiah is talking about the future restoration of Israel from their sins and salvation through Christ the Redeemer. Physical Israel in the end will not be saved through the Law but God’s grace provided to believers in Christ Jesus.
Paul also quotes from Jeremiah 31:31-34:
31" Behold, days are coming," declares the LORD, "when I will make a new covenant with the house of Israel and with the house of Judah, 32not like the covenant which I made with their fathers in the day I took them by the hand to bring them out of the land of Egypt, My covenant which they broke, although I was a husband to them," declares the LORD. 33"But this is the covenant which I will make with the house of Israel after those days," declares the LORD, "I will put My law within them and on their heart I will write it; and I will be their God, and they shall be My people. 34"They will not teach again, each man his neighbor and each man his brother, saying, 'Know the LORD,' for they will all know Me, from the least of them to the greatest of them," declares the LORD, "for I will forgive their iniquity, and their sin I will remember no more."
The Jews’ salvation will not come through following the Law, but through the declaration of the Gospel, the new birth through the Holy Spirit, and the forgiveness of sins through the blood of the Redeemer.
28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes. 29 For the gifts and calling of God are without repentance.
The Jews were now rejected, because they did not believe this Gospel. Because of this the Gentiles were added in, so the Gentiles benefited from the Jews’ rejection. However, the Jews are still a special people, who God loves and will restore. Not because of any merit of the Patriarchs, but God’s promise to them. Rather than God taking back his promise to Abraham because of the Jews’ unfaithfulness, God will fulfill His promise in saving believers, which will include national Jews in the end.
30 For as ye in times past have not believed God, yet have now obtained mercy through their unbelief: 31 Even so have these also now not believed, that through your mercy they also may obtain mercy. 32 For God hath concluded them all in unbelief, that he might have mercy upon all.
All of mankind, both Jews and Gentiles, are or were unbelievers. God’s will to have mercy extends to all. God uses even unbelief, to bring people to faith. Concluded in the Greek is sugkleio and has the sense of imprisoned. God hardened Jews and Gentiles in their sins so that He could provide mercy to everyone. This is all part of fulfilling God’s promise to Abraham. When both Jews and Gentiles believe all the nations of the world will be blessed through him.
33 O the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out! 34 For who hath known the mind of the Lord? Or who hath been his counselor? 35 Or who hath first given to him, and it shall be recompensed unto him again? 36 For of him, and through him, and to him, are all things: to whom be glory for ever. Amen.
God’s plan to bring salvation to all is a mystery now revealed to us.