Romans 11:17-25 And if some of the branches were broken off, and you, being a wild olive tree, were grafted in among them, and with them became a partaker of the root and fatness of the olive tree, do not boast against the branches. But if you do boast, remember that you do not support the root, but the root supports you. You will say then, "Branches were broken off that I might be grafted in." Well said. Because of unbelief they were broken off, and you stand by faith. Do not be haughty, but fear. For if God did not spare the natural branches, He may not spare you either. Therefore consider the goodness and severity of God: on those who fell, severity; but toward you, goodness, if you continue in His goodness. Otherwise you also will be cut off. And they also, if they do not continue in unbelief, will be grafted in, for God is able to graft them in again. For if you were cut out of the olive tree which is wild by nature, and were grafted contrary to nature into a cultivated olive tree, how much more will these, who are natural branches, be grafted into their own olive tree? For I do not desire, brethren, that you should be ignorant of this mystery, lest you should be wise in your own opinion, that blindness in part has happened to Israel until the fullness of the Gentiles has come in.
Note: Amillennialists proclaim that all Christians must be grafted into the Christian church.
BIBLICAL STUDIES IN FINAL THINGS - William E. Cox
Page 59: This passage agrees with the fact that our Lord built the church upon the faithful remnant of national Israel, and that Gentiles were later permitted to enter that same church - the same olive tree (Rom. 11:17; Eph. 2:11-22; 3:6). Paul very definitely teaches that Gentile Christians become members of the same olive tree, meaning the believing part of national Israel. Having said that some of the natural branches were broken off, the apostle went on to teach that these natural branches (Jews) would be grafted in again "if they continue not in their unbelief" (Rom. 11:23). The important point for us here is that when and if they are grafted in, the grafting will be into the same olive tree which now contains the Gentile believers. And this can only be the Christian church. This fact completely explodes the theory that God has separate future plans for national Israel and the church.
THE ISRAEL OF GOD IN PROPHECY - Hans K. LaRondelle
Page 131: Through the living witness of gentile Christians, many Jews will come to faith in Christ and thus be regrafted into the true Israel of God (Romans 11:23). Israel is thus saved in the same way as the Gentiles are saved ("so," Romans 11:26).
Setting opinions aside, the Scriptures proclaim that Christians are to be grafted into Jesus Christ. (John 15:1-8) "I am the true vine, and My Father is the vinedresser. Every branch in Me that does not bear fruit He takes away; and every branch that bears fruit He prunes, that it may bear more fruit. You are already clean because of the word which I have spoken to you. Abide in Me, and I in you. As the branch cannot bear fruit of itself, unless it abides in the vine, neither can you, unless you abide in Me. I am the vine, you are the branches. He who abides in Me, and I in him, bears much fruit; for without Me you can do nothing. If anyone does not abide in Me, he is cast out as a branch and is withered; and they gather them and throw them into the fire, and they are burned. If you abide in Me, and My words abide in you, you will ask what you desire, and it shall be done for you. By this My Father is glorified, that you bear much fruit; so you will be My disciples." Glorify the Father by abiding in the Son in the power of the Holy Spirit. Amen.