This passage is very similar in meaning and application as the passage previously discussed from Christ’s discourse in John 15. It may well be that Paul was familiar with Christ’s teaching on the Vine and the branches, and had His discourse in mind while writing about the olive tree in Rom. 11:15-24:
 For if their rejection [the Jews] be the reconciliation of the world, what will their acceptance be but life from the dead?  And if the first piece of dough be holy, the lump is also; and if the root be holy, the branches are too.  But if some of the branches were broken off, and you, being a wild olive were grafted among them and became partakers with them of the rich root of the olive tree,  do not be arrogant toward the branches; but if you are arrogant, remember that it is not you who supports the root, but the root supports you.  You will say then, “Branches were broken off so that I might be grafted in.”  Quite right, they were broken off for their unbelief, but you stand by faith. Do not be conceited, but fear;  for if God did not spare the natural branches, neither will He spare you.  Behold then the kindness and severity of God; to those who fell, severity, but to you, God’s kindness, if you continue in His kindness; otherwise you also will be cut off.  And they also, if they do not continue in there unbelief, will be grafted in; for God is able to graft them in again.  For if you were cut from what is by nature a wild olive tree, and were grafted contrary to nature into a cultivated olive tree, how much more shall these who are natural branches be grafted into their own olive tree. [NASB]
This passage of Scripture is problematic for Calvinism on multiple levels. Paul is discussing the present state of Israel throughout chapters 9-11. Calvinists find Rom. 9:6-24 to be a primary text for their doctrines of unconditional particular election and irrevocable reprobation. It is not difficult to come to such an understanding of the text when the rest of the context of Romans 9-11 is ignored. This has been the usual practice of many Calvinist exegetes. James White completely ignores Romans 9:30-33 and 11:15-32 in his exegesis of Romans 9 in The Potter’s Freedom. This is strange behavior, especially when we consider that Rom. 9:30-33 represents Paul’s conclusion to his preliminary argument in Rom. 9:1-29. Likewise, Piper (The Justification Of God) and Schreiner (Still Sovereign) neglect to give Rom. 11:15-32 any exegetical treatment in their respective works on Rom. 9 and election (Schreiner does briefly discuss 11:23, 26, but only for the sake of demonstrating that election is unto salvation in Rom. 9-11).
Paul has not changed subjects in Romans 11:15-24. He is still discussing the reprobated Jews described earlier in Rom. 9:6-24. What he says concerning these Jews is troubling to the Calvinist interpretation of unconditional election and irrevocable reprobation. Paul speaks in terms of an ancient olive tree. This tree represents the true Israel of God. It is the election of God’s people in Christ. The tree cannot represent national Israel due to the fact that many of the branches [Jews] were “broken off”. Paul is speaking of the spiritual descendants of Abraham; those who have received the promise by faith. (Rom. 4:13-25).
The unbelieving Jews have been “broken off” from the true Israel and are estranged from the promise of God’s salvation in Christ. Paul, however, holds out hope for these broken off Jews. He plainly states that they can yet be grafted in again if they do not persist in their unbelief. This truth clashes with the Calvinist belief that these unbelieving Jews have been reprobated due to an eternal and irrevocable decree. If Paul had really been teaching such a concept of reprobation in Romans 9, then he could not hold out hope for these Jews in Romans 11:23 and 11:30-32.
The further difficulty this passage poses to Calvinism is Paul’s plain declaration that those who now stand by faith may yet be broken off through unbelief:
 You will say then, “Branches were broken off so that I might be grafted in.”  Quite right, they were broken off for their unbelief, but you stand by faith. Do not be conceited, but fear;  for if God did not spare the natural branches, neither will He spare you.  Behold then the kindness and severity of God; to those who fell, severity, but to you, God’s kindness, if you continue in His kindness; otherwise you also will be cut off.
Calvinists have traditionally tried to resolve the difficulty in one of two ways. The first way is to say that the branches do not represent individuals, but nations. The broken off branches represent the nation of Israel, and the engrafted branches represent the Gentiles as a people group. The problem with this interpretation is that Paul is speaking of individual branches that have been broken off and grafted in to the true Israel of God. The branches clearly represent individual Jews, for the entire nation has not been rejected. There are believing Jews [the remnant] who have remained in the olive tree. The grafted in branches represent individual Gentiles as only believing Gentiles have come to enjoy the favor and election of God. It is only believing Gentiles that can be called spiritual descendants of Abraham, and it is beyond argument that not all Gentiles have embraced Christ.
Joseph R. Dongell provides an excellent summary:
Paul distinguishes the irrevocable call of the nation of Israel as a whole from the fate of individual Israelites. While the final destination of the people of God is absolutely certain, the future of any given individual is determined by his or her continued faith and trust in God. Gentiles who believe are grafted into the ancient tree, whereas Jews who fall into unbelief are broken off. Since faith is the sole condition for remaining engrafted, Paul issues both warning and hope. On the one hand, those Gentiles who have recently been grafted into the ancient tree through faith must humbly guard against falling into unbelief, since they too would then be severed from the tree. On the other hand, the natural branches lying on the ground can “be grafted into their own olive tree” if “they do not persist in unbelief” (Rom. 11:23-24). In other words, the destiny of God’s people as a whole is unchanged throughout the ages, though each individual branch participates in this salvation only if he or she remains engrafted by faith (cf. Jn.15:5-6). As Paul Achtemeier explains, Paul teaches destiny without teaching individual determinism. [Walls and Dongell, Why I Am Not A Calvinist, page 87]
It would seem that the interpretation of the text that would rule out the individuality of the branches is very difficult to sustain.
The second Calvinist explanation is the usual explanation that the broken off branches could only represent false converts and hypocrites who never had saving faith to begin with. This interpretation is impossible to sustain due to the fact that Paul speaks of these branches as presently standing by faith. If it is a faith that makes them “stand” then it must be genuine. It is because of their present faith that they can be said to be in the elect olive tree. Paul further confirms this when he threatens these Gentiles, who have been grafted in by faith, that they can yet be broken off from this tree if they do not “continue in His kindness”. They remain among the elect body so long as they continue in faith. If they should not continue, then God will treat them the same as the unbelieving Jews who were broken off before them. They too will be cut off “for there is no partiality with God”. If the branches that Paul threatens represented false converts, then they would have never been in the tree in the first place. How then could they be broken off?
Perhaps we should add a third interpretation. Perhaps some will say that Paul is merely presenting a hypothetical construct and threatening these branches with impossibilities. What possible effect could such a threat carry for those who could not possibly fall prey to the consequences of it? If the branches stand by faith, and those who begin in faith will inevitably continue in faith [according to Calvinism], then why warn them to continue with the threat of being cut off? If God causes believers to continue in saving faith, then to warn believers to continue is nonsensical. If God infallibly preserves the believer, and faith is a gift that we cannot help but to continue in; then to warn someone to continue in the faith would be as useless as warning someone who is hooked to a respirator to “keep breathing”.
Some will say that the warnings are God’s means by which He ensures the perseverance of His saints. Where then is the doctrine of eternal security? Can we truly be convinced that we are eternally secure, and also take the warnings of falling away seriously? If we are eternally secure then there is no danger of being cut off from the true Israel of God. If the danger is real, then there is no unconditional security. If we went to our mailbox and found a note on the door that read, “Do not open this mailbox, else a 600 pound tiger will emerge and devour you”, would we take such a warning seriously? Would such an impossible consequence truly worry us and prevent us from getting our mail?
It would seem then, that Calvinism fails to offer a valid solution to the clear teaching of Rom. 11:15-24 that those who stand by faith may yet be broken off to their own eternal ruin. Let us heed Paul’s emphatic warning: “Do not be conceited, but fear; for if God did not spare the natural branches, neither will He spare you. Behold then the kindness and severity of God; to those who fell, severity, but to you, God’s kindness, if you continue in His kindness; otherwise you also will be cut off.”