On one extreme, some believe both Israel and the church have different redemptive programs. That is, the NT church does not include OT believers. On the other, some believe the church has replaced Israel. Going by the whole counsel of the Word of God, both beliefs are found wanting.
I believe the confusion is due to lack of distinction between national Israel and true Israel. In Rom 11:1-5, Paul explained the distinction and called the true Israel as the remnant. Paul emphatically stated that God did not reject His people and he was definitely referring to national Israel for he said of himself having descended from Abraham from the tribe of Benjamin.
(Rom 11:1-5. I ask then: Did God reject his people? By no means! I am an Israelite myself, a descendant of Abraham, from the tribe of Benjamin. 2 God did not reject his people, whom he foreknew. Don’t you know what Scripture says in the passage about Elijah—how he appealed to God against Israel: 3 “Lord, they have killed your prophets and torn down your altars; I am the only one left, and they are trying to kill me”[a]? 4 And what was God’s answer to him? “I have reserved for myself seven thousand who have not bowed the knee to Baal.” 5 So too, at the present time there is a remnant chosen by grace.)
The vine tree was not replaced but only the non believing branches were removed and new branches grafted. Neither were there 2 trees growing together. On the day of Pentecost, the true Israel became the NT church and organic continuity continues till today.
Rom 11:11 clearly stated that national Israel has not fallen beyond recovery. (11 Again I ask: Did they stumble so as to fall beyond recovery? Not at all! Rather, because of their transgression, salvation has come to the Gentiles to make Israel envious. 12 But if their transgression means riches for the world, and their loss means riches for the Gentiles, how much greater riches will their FULL inclusion bring!)
Anticipating that the grafted branches (the church) will be arrogant and consider themselves superior to the broken branches (national Israel), Rom 11:17-21 warns against such arrogance.
The present hardening of national Israel is not permanent. It will be over when the times of the Gentiles are fulfilled. There will be a future restoration and they will become part of the true Israel.
(Rom 11:25-27. 25 I do not want you to be ignorant of this mystery, brothers and sisters, so that you may not be conceited: Israel has experienced a hardening in part until the full number of the Gentiles has come in, 26 and in this way all Israel will be saved. As it is written: “The deliverer will come from Zion; he will turn godlessness away from Jacob.27 And this is my covenant with them when I take away their sins.”)
In Rom 11:25-27, Paul was clearly talking about the future salvation of national Israel after the hardening is over. And our Lord Jesus will turn godlessness away from Jacob (Israel). For He will reign over the house of Jacob forever as stated in Luke 1:32-33.