This doctrine contains two sides to it. It states, in a very basic form, that those who have been elected by God to partake in salvation by the atoning sacrifice of the blood of Christ through faith in Him alone will be saved, they cannot be lost. The believer will persevere to the end in faith, though stumbling at many points along the way, being preserved by the power of God by the indwelling Holy Spirit that is given us when we believe. You cannot be elect and then be eternally lost. You are secured. This is prevalent in Scripture. (John 6:37, John 6:39)

However, in order to prevent error in people’s minds, and to not give false assurance to people who are not saved, there is a warning that comes right after this that is also prevalent throughout Scripture. This is the other side of the doctrine. It states that those who claim faith in Christ, partaking of the Holy Spirit, and even doing works in the name of Christ, that if they then fall away into unrepentant sin (or never repented to begin with), it is proof they never had true faith to begin with. This is a frightening prospect, as it should be. Jesus stated in Matthew 7:21-23, “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.'” Why is that? The warning here in the doctrine of perseverance is the warning against apostasy, a warning against falling away into sin so far that you are given over to it, hardened by sin’s deceitfulness, unable to hear the inner call of God to true saving faith, beyond the point of repentance (Apostasy – Hebrews 6:4-6).

And this warning amply applies in our day, where antinomianism seems to be running rampant in the church. Antinomianism is essentially what is commonly called Carnal Christianity. This states, in no uncertain terms, “Once saved, always saved.” Though this is true at a fundamental level (as shown at the beginning of this entry), it is very misleading. The problem is that it leads to this kind of thinking and belief: “I signed the card, prayed the prayer, so now I’m in. It doesn’t matter what I do now, I will always be secured in the love of God. I can go and live like a heathen, continue partying it up, sleep with all the women I want the rest of my life, and I will still be saved.” Wrong! But why is that? We know that our works, both good or bad, do not change our standing before the Father because of the imputed righteousness of Christ. Right? We do not earn a better standing based on our good works, and we don’t lose favor in the sight of the Father because of our sin. “Christ alone, not our works, makes us acceptable before God.” Doesn’t it seem contradictory to say on the one hand that if you’re saved you cannot lose your salvation, and then on the other hand say that if you continue in unrepentant sin you’re not saved? But here’s what it’s saying: if you claim faith in Christ, and spin off into sin, unrepentant sin where there is no struggle, living a life of rebellion to God, it proves that you never had authentic faith to begin with, and still rest under the condemnation of God. Works do not earn favor with God, but they are the natural result of authentic faith (James 2:14-26). Works without faith is dead. Faith without works is dead. On the one hand, there are unbelievers who say that their works and good deeds are their salvation, or that loving others is their salvation. And then on the other hand there are those that claim their faith is their salvation, but have no works (the natural result of faith, i.e. repentance from known sins) to back it up. If you claim Christ as Savior but you live a life of lawlessness, not even struggling with your sin, not caring for any of the things of God and His holiness, believing yourself to still be saved by what you perceive as Christ’s “fire insurance” salvation, you have reason to fear, for it may be that you have not truly believed in Christ. The true believer will persevere to the end in faith, and though they will sin and slip at times, there will be a confidence in Christ and a struggle to pursue holiness in the light of Christ, being purified by His blood, sanctified even.

The phrase, “Once saved, always saved,” is so terribly misleading, to the point of guiding people straight to hell, making people feel good about themselves while sending them straight into the furnace of God’s eternal wrath. I instead prefer the phrase, “Once saved, always changed,” because the believer will, though imperfectly, pursue holiness based upon the free grace and merit of Jesus Christ, faith alone in Christ alone. The end result of the true believer is a life set apart from the world. It is inevitable. We don’t become perfect in this life by any means, but there will be a struggle to get there. If there is no struggle, you should fear, and repent from your sin and rebellion against the almighty God, and place your faith in Jesus Christ, that He bore your sins on the cross, died, rose from the grave, ascended into heaven where He now reigns, and will come back to wipe out His adversaries and glorify Himself by glorifying His church.

Some articles pertaining to these things:

Is Repentance Necessary for Salvation? – John Hendryx
The Doctrine of Perseverance: The Earnest Pursuit of Assurance – John Piper
The Doctrine of Perseverance: The Future of a Fruitless Field – John Piper
The Lordship Controversy and Repentance – Ernest Reisinger