Why was national israel cut off ?
Paul informs us why here under inspiration why the nation of israel, that is Abraham's physical descendants of the flesh israel, why they were cut off from amongst God's People here Rom 11:19-21
19 Thou wilt say then, The branches were broken off, that I might be grafted in.
20 Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear:
21 For if God spared not the natural branches, take heed lest he also spare not thee.
What can be more plainer than this, they were cut off Because of Unbelief !
Now what did that mean according to Jesus ? Jn 10:26
26 But ye believe not, because ye are not of my sheep, as I said unto you.
Thats right, national israel was not part of the Elect Israel which was God's Sheep ! Remember when David wrote Ps 100
Make a joyful noise unto the Lord, all ye lands.
2 Serve the Lord with gladness: come before his presence with singing.
3 Know ye that the Lord he is God: it is he that hath made us, and not we ourselves; we are his people, and the sheep of his pasture.
4 Enter into his gates with thanksgiving, and into his courts with praise: be thankful unto him, and bless his name.
5 For the Lord is good; his mercy is everlasting; and his truth endureth to all generations.
Guess what ? He was not speaking of national israel according to the flesh, simply because that people israel are not the Children of God Rom 9:8, that just did not become truth when Paul wrote it in the New Testament, it was always the Truth ! God's People and Sheep Israel was always just a small remnant located in that nation !
But they believed not because they were not of God's Sheep, Covenant Israel, wherein Christ is the root and fatness thereof Rom 11:17
17 And if some of the branches be broken off, and thou, being a wild olive tree, wert grafted in among them, and with them partakest of the root and fatness of the olive tree;
See the True Israel was Chosen in Christ before the foundation, and Christ is their root and fatness, obviously indicating that was not the case for national israel, Abraham's Israel according to physical descent, since by and large they were cut off ! Also see Jn 15:6
6 If a man abide not in me [The Root of Israel], he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned.
Remember Matt 21:19
And when he saw a fig tree in the way, he came to it, and found nothing thereon, but leaves only, and said unto it, Let no fruit grow on thee henceforward for ever. And presently the fig tree withered away.
Thats national israel being cut off because of unbelief, which means they were not of His Sheep Jn 10:26
26 But ye believe not, because ye are not of my sheep, as I said unto you.484