Introduction
Many are taught "once saved, always saved". This is called "eternal security". The idea, of course, is that once you've accepted Jesus Christ as your Lord and Savior, you are saved, and you can never lose your salvation. On the surface (and in reality), this sounds wonderful! But a careful analysis of related Bible passages should cause us to be VERY CAUTIOUS about our salvation, and to NEVER take it for granted.
I will let the Bible do most of the talking here, but let me start out with a summary of what's being presented.
First, a few caveats. Number one: By no means do I claim infallibility on this subject! God forbid I should ever claim such a thing! But I have earnestly prayed for discernment and guidance in this area, and what follows is a representation of my studies to-date. It could be that I'm wrong and, in time, will come to discern that. At the moment, though, I don't believe that to be the case.
Second caveat: there are those who will say that most (if not all) of the verses which show the possibility of losing one's salvation actually only apply to those who never really had it in the first place. Now this may well be true, though I don't think the Bible clearly states that in many of these cases.
Third, by NO MEANS am I advocating a "salvation by works" doctrine!! I believe works to be a critical means for maintaining a living, active faith. It is this living, active faith which, according to the Scriptures, appears to be absolutely critical to salvation.
Okay, caveats behind us, let's move on to the exegesis.
There are two kinds of faith: "saving faith" and what we'll call "initial faith". Saving faith is the ONLY faith that will get you to Heaven. Initial faith is a faith that starts out, for all appearances, as saving faith, but then dies. Those who hold to a strict "once saved, always saved" theology will say that these were never truly saved to begin with. Regardless, this group of people, these "initially faithful" who have lost their faith (whether they were ever saved or not), are no longer (or never were) saved.
Now, here's the scary part, and the reason why I felt it was so important to get this message out. It would appear from reading the Bible and from earthly experience that those with saving faith and those with initial faith can, at the beginnings of their faith be equally as certain in their own minds that they are saved! For those with only "initial faith", this would be what's known as "false assurance".
Most recently, though, a third possibility was brought to my attention by a sister in Christ, who has provided feedback to me on a number of occasions. H.L. Nigro, author of "Before God's Wrath: the Bible's Answer to the Timing of the Rapture" and two Internet columns, "Talkin' Rapture" and "The Everyday Evangelist", provided the following insight:
"One thing to consider on the exegesis, which is something I've been wrestling
with, is whether or not someone can voluntarily give up his or her salvation.
I am a strong believer that you can never LOSE your salvation, if you want
it. But what about those who walk away, get lured away by sin, and choose
- voluntarily, knowing the consequences - not to come back. One cannot lose a
gift, but one can give it back.
This would reconcile the seeming contradictions between the eternal security
verses and those that seem to imply that you must run the race in order to win
the crown."
- H.L. Nigro
Strong Tower Publishing
My personal opinion, after digesting the Bible verses, is that each believer starts out with initial faith, and those who truly repent of their sins, who proceed to do the good works commanded by our Lord, and persevere in the faith are those who go down the path of saving faith. Or, another way to look at it might be that these people do not allow anything in this world to choke out their faith. And they do not return the gift. Saving faith endures through trials and tribulations, and through persecution. Saving faith perseveres! For an excellent presentation of faith and its development (or lack thereof), see Matthew 13:3-23. Some of these passages will be used to support the diagram below.
Here's the bottom line, then I'll give you a diagram and connect the various Bible passages to the diagram to make my case.
8 for by grace have ye been saved through faith; Eph 2:8 (ASV)
How are we saved? By grace. How do we get grace? Through faith. Okay. Now:
17 Even so faith, if it have not works, is dead in itself. James 2:17 (ASV)
How do we keep faith alive? By works (good deeds). Faith, without works is dead.
Now let's put the two parts of the equation together. To be saved, ya gotta have grace. To get grace, ya gotta have faith. To keep faith, ya gotta do works. No works, no faith. No faith, no grace. No grace, no salvation. This is NOT to say that we are saved by works! Grace is a gift of God, which NO MAN can earn! But that gift is provided through faith in Christ Jesus. This, I believe (as will be shown by many of the passages below), must be a living faith, or else the channel (or conduit, if you will) for grace to flow is gone. It is not enough to simply believe in Christ. Consider this:
Rev. Bailey Smith, in his famous sermon on "Wheat and Tares" asks the following question: "What's the difference between Satan and a Christian? Does Satan believe the Bible? Every word of it! Does Satan believe that Jesus Christ is the Son of God? Absolutely!" So where's the difference? Satan has not repented of his sins and turned his life around! That's where a Christian is (or must be) different. The initial faith and belief in Christ must be followed by repentance and living a godly life!
Your belief must be a living and active belief. The scriptures below cast serious doubt on the notion that "If I once believed in Jesus then regardless of what I believe now, I'll go to Heaven".
Okay, let's diagram the possible paths that can be taken:

The scriptures I cite below are intended to support the diagram above. I won't spend much time trying to support Paths A, B or D, since most would take these paths as "givens" and in need of no discussion (some of the scriptures will hit on these paths nonetheless). Where necessary, I've provided comments to enhance or explain the connection. Any emphasis added to the scriptures is mine. These scriptures are not presented in any particular order.
How you choose to interpret the passages above, especially when taken in total, is up to you. As for me and my household J we are choosing to err on the side of caution. One thing appears certain: a Christian life which does not bear fruit (works) is evidence of a lack of saving faith. For everyone out there who is or knows someone that feels "secure", but does not show evidence of the Spirit-filled life, please take heed! A truly born-again Christian is truly repentant, and is filled with a burning desire for God's Word, for God's Works, and for fellowship with other Christians. Any "Christian" who does not show evidence of these things is quite possibly still lost, or lost again (depending on how you interpret the passages)! If it's you, look hard at your own heart. Don't take it for granted that just because you once believed, your are eternally secure! If it's someone you know and love, help them back to the path of saving faith. This latter situation is especially critical. You have more insight into your own heart than anyone except God Himself. And you know what you need to do. But please get this message out to those you know and love who may be resting on that "false assurance", because that's one of Satan's most powerful tools! Please show them these passages, in the hopes that they may awaken the Holy Spirit within them to action, and to the path of true, saving faith, lest they go too far, deliberately rejecting the Holy Spirit and evicting their only chance for Eternal Life.
To God be the glory!