All Israel shall be saved means all the body of Christ shall be saved in the Lord. See the Church is the Spiritual Seed of Abraham who belong to Christ. Paul says to believing jews and Gentiles Gal 3:29
29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise
Galatians 3:8 And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
3:9 So then they which be of faith are blessed with faithful Abraham.
The promise was that through faith, the heathen Gentiles would be justified. Those of faith would be blessed along with Abraham.
Galatians 3:13 Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree:
3:14 That the blessing of Abraham might come on the Gentiles through Jesus Christ; that we might receive the promise of the Spirit through faith.
The blessing of Abraham found in Genesis 12:2-3 would come on the Gentiles through Jesus Christ, that the Gentiles would be justified by faith according to the promise.
Galatians 3:29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
The church, the body of Christ, is not the seed of Abraham as being considered to be his children Israel, but rather, this verse is saying we are Abraham's seed, which is referring to Christ, as can be seen in the verse below:
Galatians 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Christ is Abraham's seed and by faith we are Christ's (1 Corinthians 3:23), the children of God, and as children, heirs of God (Romans 8:17 Titus 3:7).
Galatians 3:26 For ye are all the children of God by faith in Christ Jesus.
Galatians 4:4 But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law,
4:5 To redeem them that were under the law, that we might receive the adoption of sons.
4:6 And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father.
4:7 Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ.
We do not receive the promises of inheritance given to Abraham (Romans 4:13 Galatians 3:18), but rather, we are fellow heirs of Christ according to Ephesians 3:6.