Hence, all of True Israel consists of the Gentile Christians and the Jewish Christians. Indeed, the distinction is done away with, as there is now neither “Jew nor Greek” (Gal. 3:28). All Christians, whatever their ethnic heritage, are supported by the root which is grounded in the foundation of God’s work towards our salvation in Christ—one people, unified in Christ, one tree of true Israel.
As part of this whole process, Paul also points out a couple of ways that we can see how God works—v. 22—the kindness and the severity of God. God is a God of mercy and justice. His just judgment on the Jews who do not receive Jesus as the promised Messiah, their Savior, is to cut them off the tree. As natural descendants of Abraham and the patriarchs, they had a distinct advantage due to their rich history of God’s working through their nation to bring salvation to all the world. Yet, despite all that, most Jews hardened their hearts against Christ. Hence, God’s severity is shown in his cutting them off. However, the Gentiles Christians, who weren’t even looking for the Messiah, received the gospel message and believed. Therefore, God brought Gentiles into the fold of True Israel.
I think we’ll agree that the marvelous wonder of God’s kindness cannot be overstated. What a wonder!
Even so, Paul gives a warning. In v. 22, he says that if you (referring to the Gentiles) do not continue in His kindness you will be cut off. What is up with that? Is Paul saying we can lose our salvation? Not at all. Paul’s statement is consistent with the teachings of Scripture, where the hallmark of God’s authentic people is perseverance until the end—cf. Hebrews 3:14. Further, Paul would be contradicting himself (Rom. 8:28-30; etc.).
What Paul is saying is this: If you do not, as Paul puts it, “continue in His kindness” by living according to faith, you have shown that you were never really part of True Israel. You show that somehow you never had true saving faith. Accordingly, your branch cannot be properly grafted to the tree and produce fruit and will be cut off. Jesus illustrates this concept in the parable of the sower and the seed in Mark 4.
John Calvin helps clarify this a bit in regards to individual situations:
“…bear in mind, that there are three modes of incision [that is engrafting], and two modes of excision [that is being cut off].
[First] For instance, the children of the faithful are engrafted, to whom the promise belongs according to the covenant made with the fathers; [Second] engrafted are also they who indeed receive the seed of the gospel, but it strikes no root, or it is choked before it brings any fruit; and thirdly, the elect are engrafted, who are illuminated unto eternal life according to the immutable purpose of God.
The first are cut off, when they refuse the promise given to their fathers, or do not receive it on account of their ingratitude; the second are cut off, when the seed is withered and destroyed; and as the danger of this impends over all, with regard to their own nature, it must be allowed that this warning which Paul gives belongs in a certain way to the faithful, lest they indulge themselves in the sloth of the flesh.
But with regard to the present passage, it is enough for us to know, that the vengeance which God had executed on the Jews, is pronounced on the Gentiles, in case they become like them.” 
Hence, God’s true people, the elect, the true Christians, will remain until the end.
What about God’s kindness and mercy towards the Jews? It sounds like they are getting a pretty hard deal. Not so! Verse 23 tells us God’s mercy and kindness also extends to the Jews, for if they come to believe, they will be engrafted back in. Furthermore, as natural branches, it will be so very right. C.S. Lewis, on this point, comments:
“In a sense, the converted Jew is the only normal human being in the world. Everyone else is, from one point of view, a special case dealt with under emergency conditions.”
Considering these realities, what kind of attitudes are we Gentile Christians to have? In v. 18 Paul specifically chides us not to boast against the branches that have been broken off. The history of the church proves how easy it is to fall into a type of elitism with anti-Semite tendencies. It is a temptation we must resist. We are falling in on their roots, and God isn’t finished with them. There are many of them who will later come to faith. We must not be proud and conceited, but fear God as the Judge of all the earth.