What follows is a brief discussion I had with my students in my Johannine Writings class, as we were discussing John 15 and the identity of the unfruitful and non-abiding branches (vs. 2 and 6). I ask the reader’s forgiveness, since it’s more or less strung-together notes and commentary. I added a D.A. Carson quote which I think helps a bit.
I want to offer my take on the situation, which I think would be different from all but one of you (and different from Towns, and, I believe, even Dr. Christmas). First, we need to see the particular context. In John, this really starts in John 13:31. John records a series of brief discourses by Jesus with his 11 remaining disciples, broken up only by some comment or question from one of the disciples. In each section, Jesus alludes to those who really know Him, really love Him, really know the Father, and/or distinguishing them from the world. John 13:35 says, "By this shall all men know that ye are my disciples, if ye have love one to another." Verse 36 has Peter wanting to know where the Master is going, and Jesus responds that Peter could only follow later (by following Jesus in death, due to obedience to Christ). John 14:1-4 discuss Jesus' receiving the believers unto himself, and that they know the way, which will turn out to be Christ himself (v. 6). Verses 9-14 establish that the Son and the Father are one and the same insofar as divine authority, and the works done by Christ are the Father's, and the works done by true followers of Christ will be greater because of Christ's sake. Verses 15-22 detail that doing what Christ commanded is a mark of a true believer. The apostle John would continue that theme in the epistle of 1 John; if you don't love the brethren, you aren't saved. Plain and simple. Judas, not Iscariot, wanted to know how Jesus would be able to show himself to believers, and not the world. How is such a thing possible? Because, Jesus says, he knows those who are His and in the Father, because of their fruit. And that's the context of chapter 15: distinguishing believers from the world by their fruit.
If that's the case, then we see that the most plausible interpretation, given the context, is that those in verse 2 who don't believe in Christ are eventually sifted out from the true believers: they do not possess salvation. Check out verse 24 in chapter 14: he that doesn't love Christ doesn't keep his sayings. This interpretation is also consistent with John 6:64-66, where Jesus claims that some of his larger group of disciples (followers of Jesus) were not really true believers at all! This idea of people being called disciples of Jesus without actually being saved is why the "in me" language of John 15 doesn't persuade me these are real believers. This is why I challenged some of you on the topic of the paper itself: why assume that John 15:2 is talking about the fate of unfruitful "believers," when the text only says "branches"? D. A. Carson seems to agree with this. He writes of this passage,
If we must think of ‘branches’ with real contact with Jesus, we need go no further than Judas Iscariot . . . Indeed, there is a persistent strand of New Testament witness that depicts men and women with some degree of connection with Jesus, or with the Christian church, who nevertheless . . . finally testify that the transforming life of Christ has never pulsated within them.
Similarly, verse 6 has the word "fire," which is best understood in the context of judgment. It's metaphor extending, I think, to assign the "men" in verse 6 to be someone in charge of judgment; it's plausibly not the point of the metaphor to discuss what the men are doing. So, in my view, verse 6 is most plausibly speaking of unbelievers. If one is a believer, his life will, given enough time, show some fruit of some kind. Those who do not have any fruit at all are not saved. It's a way of marking the difference between believers and unbelievers, and of underscoring the importance for believers to abide in the True Vine, Jesus. What higher goal could any of us have than to abide in Christ?!
 D. A. Carson, The Gospel According to John (Grand Rapids, MI: William B. Eerdmans, 1991), 515.