Paul is making a distinction between the gentiles that have become part of the church and natural Israel. Paul is reminding them that God has not forsaken Israel or abandoned them or forgotten His promises to them.
The Jews were blind to the truth of The Messiah. Why did this happen?
Through their blindness and refusal to accept The Messiah salvation came to the gentiles. Now Paul goes on to speak about the olive tree which represents Israel. In Romans 11:16-24 Paul explains to the gentiles, the wild olive tree, here that they have been grafted into the good olive tree, Israel, and that other branches have been cut off. The branches he is referring to are the disobedient and unfaithful branches of Israel which God allowed to be scattered.
He goes on to caution the gentiles that they should not become proud because they have been grafted in and the old branches were broken off. God did not spare the natural branches of Israel and He will not spare the new branch either, the church, if they fail to believe and obey and honor His ways. In fact Paul takes it a step further.
Paul says here that the church, the wild olive tree, is grafted into the good olive tree, Israel, which is contrary to the nature of the gentiles which comprise the church. After all they come from heathen nations with heathen cultures and do not observe that which is holy unto God. Yet God showed them mercy and made them part of the covenant and heirs to the promise through His grace and through their faith. However if they fail to keep His ways and follow His righteousness then He will cut them off just as He cut off the wicked branches of Israel. Paul also goes onto say that if God can graft people as contrary as the gentiles into the covenant how much easier must it be for Him to once again graft in the original branches, or heirs to the promise? After all they are actually branches from the good olive tree and thus it would be easy to restore them. Paul then goes on to explain, (Rom 11:25) For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in..
Now Paul is also the author of the letter to Galatia. Did he contradict himself when He wrote to the Galatians? No instead he is informing the gentile church that because of their belief and obedience to Christ they are now counted as Abrahams seed and as such are heirs to the promise equally with the righteous Hebrew saints (Gal 3:26-29). Once a gentile becomes saved and believes on The Lord as the Scriptures teach then he or she becomes part of the body of Christ as much as any Jewish believer or anyone else. Thus within the church of Christ there is neither Jew nor gentile because all are part of the same body of believers and are heirs to the promise equally, whether Jew or gentile.
Agreed and now through Christ they are heirs to the promise as well.