Before we examine John 15, I want to give a general outline of how I envision this series unfolding. We will begin by examining what I consider to be the five passages of Scripture which I believe to most clearly teach that true believers can commit apostasy (Jn. 15:1-6; Rom. 11:18-23; Heb. 6:4-8; 10:26-39; 2 Pet. 2:20-22). We will then look at the passages that are most prominently used by the advocates of inevitable perseverance to see if they truly teach that doctrine. Lastly, we will look to discover which understanding of perseverance best conforms to what the Bible teaches regarding assurance of salvation.
[All quotes are from the NASB unless otherwise stated]
John 15:1-6 reads as follows:
 I am the true vine, and my Father is the vinedresser.  Every branch in Me that does not bear fruit, He takes away; and every branch that bears fruit, He prunes so that it may bear more fruit.  You are already clean because of the word I have spoken to you.  Abide in Me and I in you. As a branch cannot bear fruit of itself unless it abides in the vine, so neither can you unless you abide in Me.  I Am the vine, you are the branches; he who abides in Me and I in him, he bears much fruit, for apart from Me you can do nothing.  If anyone does not abide in Me, he is thrown away as a branch and dries up; and they gather them, and cast them into the fire and they are burned.
Jesus is speaking directly to His disciples who are already “in Him”. They are “clean” (pruned). Their present status is not in question. They are branches attached to the true vine (verse 5). It is very important to understand that Jesus is speaking to saved individuals. They have life because they are attached to the source of life. Jesus is not talking about how one comes to be in Him (get saved). He is speaking of the importance of abiding in Him. Young’s Literal Translation renders “abide” as “remain”. It can also be understood as “continue”. The branches in the true vine must remain in Him in order to continue to enjoy the life that flows from Him. No one can have life outside of Christ. The believer remains in Christ through faith and will continue to produce the fruits of faith and life for as long a he or she remains in Christ. When a branch ceases to remain (through faith), as indicated by fruitlessness, it is cut off. Here is a vivid and concise picture of the nature of apostasy. The apostate is not someone who was never in the vine, but someone who did not remain in the vine. Only true believers can be said to have genuinely been in the vine. No unbeliever can be said to be “in Christ”.
This passage undercuts the Calvinist definition of apostasy. Jesus is not speaking of those who had never been in Him. He is not speaking about the visible church. He is speaking about those who are in the true vine, which is Christ Himself: “I am the true vine”. The branches in the true vine can only be true believers. False professors can never be said to be in the true vine.
The Calvinist is correct to say that the branch which is cut off represents an unbeliever. The relevant question is not whether or not the branch that is cut off is an unbeliever, but whether or not the unbeliever had previously been a believer in the “true vine”. It is impossible to conclude otherwise when we allow Christ to define His own terms.
The Calvinist objection cannot be sustained for the following reasons:
1) Jesus defines Himself as the true vine and the branches as being “in Me”. Robert Shank well points out the absurdity of insisting that Jesus is only speaking of Himself as the visible church:
Unable to deny that ‘branches’ defect and are cast forth, the proponents of unconditional security find themselves under the necessity of ‘defining’ the branches. Bishop Ryle therefore contends that “…it cannot be shown that a ‘branch in Me’ must mean a believer in Me. It means nothing more than ‘a professing member of My Church, a man joined to the company of My people, but not joined to me.’” Such a contention is necessary, of course, if one is to defend the doctrine of unconditional security. But some of us find it difficult to conceive of Jesus as saying to His Apostles, ‘I am the vine, and all who are professing members of my Church and joined to the company of my people though not necessarily joined to Me, are the branches in Me.’ (Life In The Son, pg.45)
He then quotes another such “definition” of the branches:
Similarly, Hengstenberg quotes Lampe as saying, ‘In a certain sense, even hypocrites may be said to be in Christ, partly because, in the external fellowship of the Church, they partake of the sacrament of union with Christ, and therefore boast themselves of being in Christ; partly because they are esteemed by others to be such as belong to the mystical body, or at least are tolerated in the external communion of the disciples.’ But again, it is difficult to conceive of Jesus as saying, ‘I am the vine, and all who partake of the sacrament in the external fellowship of the Church and who therefore boast themselves of being in Me and are esteemed by others to be such as belong to the mystical body, or at least are tolerated in the external communion of the disciples, are the branches.’ (ibid. 45 emphasis his)
He finishes by quoting John Calvin:
Similarly, in an attempt to reconcile the passage with his theology, Calvin declares that ‘…many are supposed to be in the vine, according to the opinion of men, who actually have no root in the vine.’ True; but irrelevant. For Jesus was not speaking about the opinions of men, but about solemn realities- about things as they are, not as men may imagine them to be. We protest that any definition of the branches that cannot easily be inserted into the Saviour’s discourse without a sense of glaring incongruity is obviously inadmissible. And again, it is unthinkable that Christ should say, ‘I am the vine, and all who are supposed to be in the vine according to the opinion of men, some of whom do not actually have root in the vine, are the branches.’ (ibid. 45, 46)
I am in full agreement with Shank’s conclusion:
Such arbitrary definitions of the branches, ridiculous as they are, are nevertheless unavoidable for all who deny that Jesus taught that men who are true believers can ultimately abandon the faith and fail to abide in Him, thus to be cast forth and withered and, in the end, burned. (ibid. 46)
2) Jesus is speaking of those who cease to “remain” in Him, and not those who were never in Him in the first place. It is absurd to think that a branch can be cut off from or cast away from a vine that it was never in.
3) Jesus is directly addressing His disciples who were truly saved.
4) The branch that is “cast forth” from the vine is said to “wither” or “dry up” before being cast into the fire. It is meaningless to speak of an already dead and withered branch (such as would be the case of a hypocrite or false convert) drying up or withering. Such things are only spoken of branches that once possessed life. The fact that the branch withers is a clear indication that it once possessed life. The only way that the branch could have once lived was through being attached to the vine (Jesus Christ- the only source of spiritual life).
Conclusion: It would seem then that if we allow the text to speak for itself, we must submit to the reality of apostasy. We must also conclude that the apostasy spoken of in this passage has reference to true believers abandoning the faith and being removed from the vine. The Calvinist conception of apostasy (leaving something that you were never truly a part of) is incompatible with the plain language of Jesus’ discourse. Robert Shank has well said, “Let us accept at face value our Saviour’s grave and loving warning that it is indeed possible for us to forfeit eternal life by failing to abide in Him ‘who is our life.’” (ibid. 46)