The Apostle Paul utilizes a horticultural analogy in the book of Romans which and the interpretation of which has become highly disputed:
If the dough offered as firstfruits is holy, so is the whole lump, and if the root is holy, so are the branches. 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, 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. Then you will say, “Branches were broken off so that I might be grafted in.” That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear. For if God did not spare the natural branches, neither will he spare you. 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. 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. 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.11:16-24
There are essentially three interpretations of this passage. The Reformed have an interpretation which supports infant baptism and Particular Baptists have a variety of reasons for disagreeing. But there is a very popular third interpretation which cannot be dismissed a priori when deciding between the Particular Baptist and Reformed readings. The third interpretation is that Romans 11 teaches that people can lose their salvation; that a person can be savingly united to Christ, and then cut off, undoing his justification and surrendering his salvation.
I want to offer two reasons why this is an impossible interpretation, and I will do so without presupposing Perseverance of the Saints. In other words, I will not argue that Romans 11 should not be read this way because “other passages teach we cannot lose our salvation.” Rather, I will even, for the sake of argument, presuppose that justified believers can lose their salvation. Even if that doctrine is true, Romans 11 cannot be used to support it.
1) Regrafted Branches
This interpretation would contradict what the Scriptures elsewhere teach about losing salvation, provided one interpreted all the relevant passages consistently. Consider Hebrews 6,
For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.4-6
Hebrews 6 is clear that those who fall away can never be restored; they can never be saved again. Compare this to Romans 11 which says,
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. 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.23
In Romans 11 branches which were cut off can be grafted back in to the tree. If union with the branch makes one saved, then this passage not only teaches a person can lose their salvation, but that they can regain their salvation, something which Hebrews 6 more plainly declares to be impossible.
2) Natural Branches
Most importantly, this interpretation would lead to the conclusion that Jewish people are born saved. It would force the interpreter to deny Original Sin, believing that the Jews are born saved and without need to believe in Christ for forgiveness.
If union with the root is salvation, then what does one do with the fact that there are natural branches? Not every branch is grafted into the root, the natural branches grow from it – they never knew a time when they were not united to the branch. This means these branches were “born saved.” Since original sin has been so clearly established by all parties, since no one maintains a person can be born saved, this interpretation can be dismissed.
This is also relevant to the notion of holiness. Perhaps one of the reasons many are so quick to believe union with the root mediates salvation is because of the word “holy” (… if the root is holy, so are the branches (16)). Yet, the New Testament can speak of a person as holy without our assumption that such a person is justified:
For the unbelieving husband is made holy because of his wife, and the unbelieving wife is made holy because of her husband. Otherwise your children would be unclean, but as it is, they are holy.(1 Corinthians 7:14)
Yet again, to interpret holiness as justification, one must conclude from this that some children are born saved. This reductio ad absurdum then must compel us to reject a reading of Romans 11 wherein the severed branches represent Christians losing their salvation.