The olive tree represents Israel. In Paul's discussion, the natural branches were the Jews. Some of these were "cut off" because of unbelief, and wild olive branches, the believing Gentiles, were grafted into Israel. Paul ended with a warning about pride against the Jews. If God would cut off the unbelieving Jews, the same could happen to the Gentiles if they fell into unbelief.
Romans 11:22-26 (KJV)
Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off.
And they also, if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again.
For if thou wert cut out of the olive tree which is wild by nature, and wert graffed contrary to nature into a good olive tree: how much more shall these, which be the natural branches, be graffed into their own olive tree?
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.
And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:
In verse 22 Paul pointed out the severity of God that came upon the Jews because of unbelief. In contrast, God had been exceedingly good to the believing Gentiles by grafting them in. He urged the Gentile believers to continue in God's goodness, or otherwise, they would also be cut off. Remaining in God's goodness is accomplished by staying in relationship with Him, by continuing in faith. The alternative is to be cut off just as the unbelieving Jews had been.
But Paul didn't end there. Again he declared hope for the Jews (verse 23). If they didn't continue in unbelief, they too, would be grafted back into the olive tree.
For if the Gentiles, who believed, could be grafted into the olive tree, even though this was not their native tree, how much more easily could the cut off branches be grafted back into their own tree (verse 24)?
In verse 25 Paul spoke about not wanting his fellow believers to be ignorant of God's plan, because without understanding they might be conceited. Although Paul stated very clearly that a blindness had happened to the Jews and that it would continue until the fullness of the Gentiles had come into the Kingdom of God, Christians today act extremely conceited in their belief and the unbelief of many of the Jews. Simply put, God is not finished with the Jews! It would appear that the blindness that came on the Jews also has affected much of Christianity. Although there is belief, there has been much shunning of the Jews and Israel. Christians don't really seem to realize that they have been grafted into a very Jewish olive tree.
However, the blindness is not complete. Jews have come and are coming to faith in Messiah Yeshua. In the same way, Gentiles, for centuries, have been coming to faith. The blindness is going away! Eventually, as the full number (or more likely, the full quality or kind) of Gentiles comes into the Kingdom, all Israel will be saved (verse 26)!
Paul then quoted from Isaiah.
Isaiah 59:20 (KJV)
And the Redeemer shall come to Zion, and unto them that turn from transgression in Jacob, saith the LORD.
One day, the nation of Israel will recognize Yeshua, accept Him as Messiah, and turn from transgression. This is the outcome of Paul's discussion that began with his concern over his fellow Jews. How this must have comforted him!