Today, all of those who are in Christ are now Israel.
In the OT "all Israel" were all of the children of Israel, all of the congregation of Israel, or all of the tribes of Israel, or all the house of Israel. Also synonymous with the seed of Jacob, or all the seed of Israel, or the seed of Isaac, or the seed of Abraham. Some examples are:
"They saw not one another, neither rose any from his place for three days: but all the children of Israel had light in their dwellings."
"And all the children of Israel murmured against Moses and against Aaron: and the whole congregation said unto them, Would God that we had died in the land of Egypt! or would God we had died in this wilderness!"
"These be the words which Moses spake unto all Israel on this side Jordan in the wilderness,...."
Psa. 22:23 -
"Ye that fear the Lord, praise him; all ye the seed of Jacob, glorify him; and fear him, all ye the seed of Israel."
1 Chron. 16:13,
"O ye seed of Israel his servant, ye children of Jacob, his chosen ones."
"I have not spoken in secret, in a dark place of the earth: I said not unto the seed of Jacob, Seek ye me in vain: I the Lord speak righteousness, I declare things that are right."
Isa. 45:25 -
" In the LORD shall all the seed of Israel be justified, and shall glory."
"But the house of Israel will not hearken unto thee; for they will not hearken unto me: for all the house of Israel are impudent and hardhearted."
Under the old covenant they were the physical blood line and any other who joined with them.
"And the children of Israel, which were come again out of captivity, and all such as had separated themselves unto them from the filthiness of the heathen of the land, to seek the Lord God of Israel, did eat,"
But, under the new covenant there was a change.
"24 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. 25 But after that faith is come, we are no longer under a schoolmaster.
26 For ye are all the children of God by faith in Christ Jesus.
27 For as many of you as have been baptized into Christ have put on Christ.
28 There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus.
29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise." (KJV)
“And think not to say within yourselves, We have Abraham to our father: for I say unto you, that God is able of these stones to raise up children unto Abraham.” (KJV)
John was telling the Pharisees and Sadducees, the sects of the Judeans (Jews), that their fleshly lineage to Abraham, their genealogy was not the deciding factor in whether they were worthy. Then something else was needed to be considered worthy.
“Bring forth therefore fruits meet for repentance:”
If you are going to be worthy of the inheritance, to be considered as children of God, of the house of Israel, then you have to be obedient and faithful under the gospel of Christ.
“Blessed is the man to whom the Lord will not impute sin.
9 Cometh this blessedness then upon the circumcision only, or upon the uncircumcision also? for we say that faith was reckoned to Abraham for righteousness.
10 How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision.
11 And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also:
12 And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised.
13 For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.“
It is the faithful that are the House of Israel. The circumcised Jews thought they were so special that all the rest of mankind was cursed, and not to be considered equal to them. The message that John and Christ brought in the fullness of time during the first century A.D. was that the faithful, those who followed after our heavenly Father were of the House of Israel.
“Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all, 17 (As it is written, I have made thee a father of many nations,) before him whom he believed, even God, who quickeneth the dead, and calleth those things which be not as though they were. 18 Who against hope believed in hope, that he might become the father of many nations, according to that which was spoken, So shall thy seed be.”
This idea that all of mankind, that all nations of the world were invited, were called to come back to God was inconceivable to many of the Jews. They had misunderstood the covenant relationship with God from the very beginning of that covenant given in the exodus wilderness.
They were to be a kingdom of priests, a holy nation (Ex. 19:6), an example unto all of the other nations on earth of how to be acceptable to God. They were to be holy, a wife honoring one husband, and a mother to the other nations on earth.
Instead, the Israelites became proud, snapped their suspenders, believing they were the only people on earth whom God loved. Jesus made a point of telling those in Nazareth that there were others outside of Israel who had received God’s blessings when none in Israel had. (Luke 4: 25-27)
It was for the promise God made to Abraham - for Abraham’s sake - that God used the children of Abraham, through Isaac and Jacob to bless all nations of the earth. (Gen. 26:24; 28:4; Ex. 32:13; Is. 41:8; Acts 3:25)
It was never for the sake of the children of Israel, but for the sake of Abraham! Abraham’s faith was reckoned unto him for righteousness! (Gen. 15:6)
“ *Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
8 That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.”*
“As he saith also in Osee, I will call them my people, which were not my people; and her beloved, which was not beloved. 26 And it shall come to pass, that in the place where it was said unto them, Ye are not my people; there shall they be called the children of the living God.“
“ What shall we say then? That the Gentiles, which followed not after righteousness, have attained to righteousness, even the righteousness which is of faith. 31 But Israel, which followed after the law of righteousness, hath not attained to the law of righteousness. 32 Wherefore? Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumblingstone;”
No one's DNA, no one's blood line matters anymore. The call of the gospel of Christ is open to everyone of every nation and tribe on earth. Everyone who believes and is baptized into Christ, puts on Christ, and becomes of the seed of Abraham.
Rom. c. 11 comes after the teaching of chap. 4-10 that the faithful were always the Israel of God, and begins by making the distinction that fleshly Israel were not being cast away.
"I say then, Hath God cast away his people? God forbid. For I also am an Israelite, of the seed of Abraham, of the tribe of Benjamin."
Paul then separated those unfaithful from the remnant that were faithful.
"Even so then at this present time also there is a remnant according to the election of grace."
The grafting in of the nations (gentiles) through the broken branches of the tree (the unfaithful and unbelieving children of Israel) were the same opportunity for the call of the tribes of Israel to return to God.... through the gospel of Christ.
" Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off. 23 And they also, if they abide not still in unbelief, shall be grafted in: for God is able to graft them in again."
So, then under the gospel of Christ all believers, both the original branches (the blood line of Abraham, Isaac, and Jacob) and the grafted branches (all the nations) are become one in Christ, and are counted for the seed.
And, Rom. 11:26 refers to the joined branches of the tree as "all Israel". Fleshly Israel had the same opportunity to be in Christ as all of the nations do. This is the gospel that was preached from the beginning.
"And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice."
"And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation;"
“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. 9 So then they which be of faith are blessed with faithful Abraham.”
“ 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.“
FURTHERMORE, we must remember the first "milk" of the word when studying the heavier "meat" of the word (Heb. 5:13).
"He that believeth and is baptized shall be saved;..."
and, Rom. 11:26,
"And so all Israel shall be saved:..."
Both of these statements are true, as the Holy Spirit does not contradict Himself. Therefore, he that believeth and is baptized (into Christ) shall be saved, and all Israel shall be saved joins the two, and makes them the same. The definition of "all Israel" are those that believe and are baptized into Christ.
Christ is the seed of the promise made to Abraham. It is through Christ that we can become children of Abraham, by faith!
So, the house of Israel were always those of the faithful of God, those who followed after the spirit of God, and are sons / children of God. (Roms. 8:14)
Therefore, when God said that he would make a new covenant with the house of Israel (Jer. 31:31, 33), then the new covenant promise is under the gospel of Christ, and the Israel that is in Christ.
Under the old covenant the house of Israel were the physical children of Jacob, all those that could trace their genealogy back to their physical ancestor, Abraham. When we are reading the Old Testament, we know that the references to the house of Israel were speaking of the tribes of Israel, the descendants of Jacob.
But, under the new covenant the definition of the house of Israel was fully completed - fulfilled - and changed from the blood line to the promise. All those that have been baptized into Christ are now the line of the faithful!
So the new covenant that He made with the house of Israel is with all those that are in Christ, and the Israel which is in Christ is now saved with an everlasting salvation. (Isa. 45:17)
Today, the house of Israel are spread throughout all nations of the world where ever they are, whomever have been baptized into Christ. Just as God promised Abraham that his seed should be multiplied as the stars of heaven, so they are! (Gen. 22:17; 26:4; Ex. 32:13)
(All scripture is from the KJV. All bold emphasis is mine.)