Delimiting the Text
17 But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree,
18 do not be arrogant toward the branches. If you are, remember it is not you who support the root, but the root that supports you.
19 Then you will say, “Branches were broken off so that I might be grafted in.”
20 That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear.
21 For if God did not spare the natural branches, neither will he spare you.
22 Note then the kindness and the severity of God: severity toward those who have fallen, but God’s kindness to you, provided you continue in his kindness. Otherwise you too will be cut off.
23 And even they, if they do not continue in their unbelief, will be grafted in, for God has the power to graft them in again.
24 For if you were cut from what is by nature a wild olive tree, and grafted, contrary to nature, into a cultivated olive tree, how much more will these, the natural branches, be grafted back into their own olive tree.
The roots and branches analogy started in verse 16 is filled out here in verses 17-24. Paul uses the metaphor of an olive tree to illustrate the necessity of faith and God’s sovereignty in saving. There is apparently a tension between Jews and Gentiles simmering in the background (see Romans 14-15) that manifested itself in the form of Gentile boasting (11:19).
In response, Paul gives two reasons against Gentile boasting. The first, in verses 17-18, is that the Gentiles were not the original recipients of the promises. The Gentiles are grafted into the promises given to the patriarchs. They are only recipients of the promises because some of the branches (Israelites) were broken off. They cannot boast that they were saved because of their ethnicity, that would be to nullify the salvation that comes through faith alone. God’s sovereign choice, as demonstrated back in chapter 9, was to choose recipients of the promise based on his own will. Therefore, the Gentiles cannot boast that they were saved because they were better or more deserving than the Jews. The promises of God are upheld with respect to the Gentiles because they partake in the promises given to the patriarchs.
Verses 19-20 anticipates a Gentile objection, that some of Israel were accursed and cut off for the sake of the ingrafting of the Gentiles. Paul does not deny it; it is true that the unbelief of Israel was orchestrated by God for the sake of the salvation of the Gentiles. He says the same thing himself in 11:11. However, Israel was not broken off in spite of their faith and belief, they were broken off because of their lack of faith and unbelief. The graft that holds the Gentile branches in place on the tree of the people of God is held only by faith. Without faith, all branches, grafted or not grafted, will fall. In verses 21-22, Paul argues that the Gentiles’ response should not be boasting over their triumph. Rather, they should fear, because if they were to stop exercising faith, they too would be cut off. In an argument from the greater to the lesser, Paul points out that if God could cut off the apostate native branches, he would be all the more willing and able to cut off the apostate grafted branches. The Jews could not escape the judgment of God when they became apostate – similarly, the Gentiles will not escape judgment either if they become apostate.
It should be noted that God’s rejection of the Jews does not withhold his desire to save. Israel’s failure is a function of their lack of faith, yet if they have faith, they will be grafted in. Verses 23-24 is another argument from the greater to the lesser – if God can be supposed to have grafted wild olive shoots into a tree, then how much more possible it will be to graft the native branches that have been cut off. The casting off of Israel is not irreversible, and is indeed desired by God.
The qualifications of verses 22 (“provided you continue in his kindness”) and 23 (“if they do not continue in their unbelief”) show that there is an intimate linkage here between the exercise of faith and God’s favor. As Murray astutely comments, “There is no such thing as continuance in the favour of God in spite of apostasy; God’s saving embrace and endurance are correlative” (Romans II, 88).
 While specifics of the analogy may seem odd, such as the practice of breaking olive branches and then re-grafting them into the same tree, the point of the analogy should not be missed. Murray points out that Paul’s familiarity with the details of contemporaneous horticulture practices do not negate the basic outline of the analogy. Do olive shoots really matter that much? Could not the same point have been made with oranges or apples?