Rom 9:6 But it is not as though the word of God hath come to nought. For they are not all Israel, that are of Israel:
The larger set of genetically Jewish people are not all the same set of people known as Israel spiritually. This doesn’t make statements regarding those who are genetically Gentile.
They might be better pictured as the grafted branch.
But genetic believing Jews and genetic believing Gentiles make up the Church, no? Together they comprise the true olive tree (Rom. 11). Paul's thesis in Rom. 9-11 is that the remnant of Israel is the true Israel. And elsewhere he makes it plain that Gentile believers are made part of that same believing commonwealth (Eph. 2). They are entitled to all the right and privileges as the natural branches. Paul treats them equally in every respect (Gal. 3:28,29). Continuing to refer to them as the grafted branch, as if that is inherently spiritually inferior, is not helpful. Remember, we are 2000 years past the point where Paul was with the fledgling Church. The new man has continued to grow to maturity. We are no longer simplay a new man, but a mature man. We have put away childish things.
Sadly, there are those today who wish to take that mature man back to the days when believers were finding offense with one another over food and feast days. To the days when even apostles were withdrawing from the gentile brethren when the other party arrived. Back to the days of "I am of Paul," or "I am of Apollos," or "I am of Cephas,"
Now, you can make clams that “The Church is not Israel”, and at some level that may be true, but at another level, the important spiritual level, is is not true. The Church is Israel, the holy nation and royal priesthood (1 Peter 2:9,10).