Warning: I'm not advocating what I post in the next paragraphs, I'm only putting it out there as a possibility, as something to be considered. I just haven't thought enough about this proposal to dismiss it or promote it, but I do find it interesting and potentially helpful.
There are several troublesome passages throughout the New Testament, especially in Hebrews, but not limited to Hebrews, which offer stern warnings regarding the danger of apostasy. See, for example, Heb. 6:1-12, Heb. 10:26-31, 1 Cor. 15:2, 2 John 7-8, and also passages like Matt 10:22, 2 Tim 2:11-13, . How do we reconcile those passages with the seemingly contradictory claims of of the Bible that saints are eternally secure and will persevere (be preserved) till the very end? This encouragement comes from nearly all corners of the NT - from Peter (1 Peter 1:5),Paul (Rom. 8:30, Eph. 1:13-14, Phil. 1:6), John (1 John 5:13), and Jesus (John 6:38-40, John 10:27-29). How do we hold these things together in proper tension?
Some, from the Arminian/Wesleyan theological camp, argue that the warnings are indeed warnings to true Christians that they can forfeit their salvation through apostasy or continuing patterns of sin (some going so far as to say you loose your salvation every time you commit a willful sin). Thus, the warning passages are given tremendous weight while the comforting passages which emphasis security are given short shrift. Obviously, that is overly simplistic, but serves to set the contrast of the other views.
Some theologians from the Reformed standpoint (including Baptists) tend to emphasize the security side of the equation, explaining away the warnings in various ways. Some explain away the warnings of Hebrews as hypothetical warnings - "if you, as a believer, were to turn away from the faith, this would happen. If you, as a believer, were to keep on in your sinful ways, you'd be in deep trouble. It's not possible, but for the sake of argument, if it did, there'd be no hope for you." Others from Reformed camp explain the warnings against apostasy away arguing that those who fall away weren't really and truly Christians at all. They may have been upstanding members of the local church, but not of the invisible church. They weren't truly Christians, but only appeared to be so.
There's other variations on those above positions. I don't think I've ever held the first (Wesleyan/Arminian) position - not even in my unReformed wandering years. The second position which was presented only in brief (and caricature form) above has been/is my position. Today (5/31/11), however, I heard a third position that I think demands some attention. Doug Wilson argues that you can't simply explain away the warning passages, but you can't neglect those passages which promise security either. The solution: realize that the Bible is speaks in two different ways about being a Christian.
There are those who are apart of the covenant community of the church. They are Christian in this sense - they bear the marks of the covenant, meaning they have been baptized and partake of the Lord's Supper. They participate in the corporate life of the covenant community.
But, there is another sense which we can't ignore if we are to make sense of the biblical tension. This other sense we can term the 'decreetal' sense. Those who are Christians in the decreetal have been elect (predestined) before the foundation of the world, find their way into the covenant community by God's providence and are kept from falling by God's providence. They are 'in Christ' by God's eternal decree.
Here's why understanding these two senses is important. Those who are Christians in the covenantal sense are truly Christian in some sense, but not necessarily Christian in the decreetal sense. Therefore, when they are warned against falling away, it is a genuine warning. Don't loose what you have in the covenant community. If you do, you've lost something real and valuable. Those who believers in the decreetal sense cannot fall away, but since we aren't privy to the council and decrees of God, everyone must take these warnings seriously. Falling away is proof that though one may be a Christian in the covenantal sense, they were certainly not one in the decreetal sense.
To support his view, Wilson points again to two different kinds of metaphors in the Bible. There are those that talk of believers and unbelievers as 'ontologically different'. So, Peter can refer to unbelievers as sows and dogs (not sheep). Jesus can refer to them as goats (not sheep), or as tares (not wheat). Though they can be washed up, they aren't changed from a pig to a sheep; though they are in the same field, they aren't of the same seed. These metaphors which point to the ontological difference between believers and nonbelievers is viewing them from a decreetal perspective. Some are elect, others are not.
On the other hand, other metaphors can view believers and unbelievers as of the same stock, but different with regards to fruitfulness. So Jesus can refer to branches being broken off and tossed aside. There isn't a distinction between the branches kept and those lopped off except that one is fruitless and the other fruitful. This is a view from a covenantal perspective.
I want to do a lot more thinking and reading on this perspective. As I see it now, this understanding seems to 1) value the church as the covenant community - it is something valuable and real, 2) take the warnings against falling away seriously, 3) fits those warnings with the passages promising preservation by God in a healthy way. This understanding does flow from a controversial movement called Federal Vision, hence my reservations about recommending it wholesale. I'm honestly out of the loop on this, but it's a hot button issue in the Presbyterian Church of America. I'm open to thoughts, suggestions for further readings, push backs, etc.
Here's a short interview with Doug Wilson on this issue: