Can you lose your salvation? A number of people, even many true Christians, would answer that yes, you can. There are imaginary lines they draw in their minds which, if crossed, result in the saved becoming unsaved. You were going to Heaven, but then you did this thing too many times, or you did that thing even once, or God forbid you held that errant view that doesn't really have much impact on the Gospel! And this varies radically between different believers. It's called "Conditional Security". Some believers are fairly confident that there's almost nothing you can do to lose salvation, but there is a limit. Others believe it's so broad that even eating meat on a Friday can be a "mortal" sin. Nevertheless, they believe salvation can be lost in one way or another.
Reading through the book of 1 John, I don't see how any Christian can honestly hold to the view that one can lose salvation. John literally expresses his purpose as being so that those who believe in the name of the Son of God may know that they have eternal life, and that they may continue to believe in the name of the Son of God. In other words, the Gospel is permanent. Jesus didn't get you part of the way there, then took the stabilisers off your bike and told you to pedal the rest of the way.
A simple question to ask yourself is how did you receive salvation? Through faith! It wasn't anything you did, or that you didn't do, that bought you salvation, and so how can it be anything you did, or didn't do, that kept it?
As a Christian, everything that you do should be a direct result of your faith. The Bible assumes that, one way or another, you are still going to sin. It never says "Great, you believe now, so you're saved, but if you do this, that and what not, you're going straight back on the path to Hell." In fact, 1 John 1:8 says that if you say you have no sin, you're deceiving yourselves, and the truth is not in you. But the Bible also lays out the solution for that. Another purpose John lays out for his first epistle is this: "My little children, these things I write to you, so that you may not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous. And He Himself is the propitiation for our sins, and not for ours only but also for the whole world." (1 John 2:1-2).
In other words, yes, you're still sinful. It's not the Christian who sins who's going to Hell, it's the "Christian" who thinks he isn't a sinner. But the answer to all of that is Jesus. He is our advocate with the Father, He is the propitiation for our sins, it is our belief in Him that saves us, not our deeds. And so we can know we have eternal life, which would not be a possibility if we could lose our salvation, because we believe in the name of the Son of God.
Now, if we can't lose our salvation even through sin, can't we just sin and let Jesus deal with that? In theory, yes. But the Bible assumes that the true believer won't even want to. If you're the kind of person who does use the grace of God as an excuse to sin, you're not going to lose salvation because you don't even have salvation to lose. Faith is more than just the intellectual assent that Jesus exists and died on the cross and rose again. If you truly believe that sin is bad, you're going to treat sin as if it is bad. If you really love Jesus, you're going to obey His commands. 1 John 5:3 says "For this is the love of God, that we keep His commandments. And His commandments are not burdensome." (Emphasis mine).
Going into the third chapter of 1 John, we read the following: "Whoever commits sin also commits lawlessness, and sin is lawlessness. And you know that He was manifested to take away our sins, and in Him there is no sin. Whoever abides in Him does not sin. Whoever sins has neither seen Him nor known Him. Little children, let no one deceive you. He who practices righteousness is righteous, just as He is righteous. He who sins is of the devil, for the devil has sinned from the beginning. For this purpose the Son of God was manifested, that He might destroy the works of the devil. Whoever has been born of God does not sin, for His seed remains in him; and he cannot sin, because he has been born of God." (v4-9)
That's some very deep cutting stuff. In fact, it is so deep cutting that it broke me out of the most sinful time in my entire Christian life. It was shortly after a very painful betrayal, I was angry with God for letting me endure such a thing, I was in constant mental anguish, I used sin as a comfort blanket and a distraction. But when I read that passage, it shook me to the very core, because I understood that whatever pain I was in, however angry I was at this person, anger at God was utter foolishness, and I was not abiding in Him at that time. Now, evidently I was still faithful to Him. I had my nose in His word, I was seeking to reconnect with Him (and thanks be to God that He was merciful enough to do that), but I read that and I realised that as much as I was claiming to be His son, I certainly wasn't acting like it.
But what happened? As a result of realising my folly, I started to flee from it, towards God. Had I been an unbeliever, or a fake Christian, that wouldn't have happened. I wouldn't have cared. If I'd have read that, it wouldn't have fazed me, and I'd have continued in my sin. This is because salvation isn't earned by good works, it generates good works. Salvation isn't kept by avoiding sin, it causes one to avoid sin. As my old pastor once said, "Christianity is about being done with sin, even if sin isn't done with me."
This statement is a wonderful summary of Romans 7:14-24. "For we know that the law is spiritual, but I am carnal, sold under sin. For what I am doing, I do not understand. For what I will to do, that I do not practice; but what I hate, that I do. If, then, I do what I will not to do, I agree with the law that it is good. But now, it is no longer I who do it, but sin that dwells in me. For I know that in me (that is, in my flesh) nothing good dwells; for to will is present with me, but how to perform what is good I do not find. For the good that I will to do, I do not do; but the evil I will not to do, that I practice. Now if I do what I will not to do, it is no longer I who do it, but sin that dwells in me. I find then a law, that evil is present with me, the one who wills to do good. For I delight in the law of God according to the inward man. But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members. O wretched man that I am! Who will deliver me from this body of death? I thank God—through Jesus Christ our Lord! So then, with the mind I myself serve the law of God, but with the flesh the law of sin."
I often marvel that God saves any of us at all. Looking at my own life, I can certainly echo Paul's words. "For the good that I will to do, I do not do; but the evil I will not to do, that I practice." Why? Why must I put off these good things I could have, and should have done? Why must I do these evil things I can obviously avoid, but in my stupidity, I do them anyway? Why can't I just be like Jesus? Our flaws are natural, but are not an excuse. But this is the point of salvation. The unsaved do not care that there are points in their lives where they fall short. Even the sins that they admit are sins are somehow "not that bad". God will still let them into Heaven, right? I mean, God's a good God, they're good people, why should it matter that their language is atrocious, that they're consuming drugs of various kinds, that they're sleeping around and watching porn, that they're dishonoring their parents, why should any of this matter?
But it does matter, and as believers, we recognise that, and so because we are saved, we serve the law of God with our minds, and yes, unfortunately our flesh will continue to serve the law of sin until it is destroyed. But once it is destroyed, we have this glorious guarantee: Christ died to save sinners. And that wasn't a pitiful, partial sacrifice. It was a perfect one. Every sin you have ever committed, every sin you are committing, and every sin you will ever commit, is dead. And so you won't be.
All of this is to say that salvation isn't about your own glory. It doesn't depend on you, but on Him. You are not worthy of salvation. In fact, that is the very definition of salvation. If you did anything to keep it, that's something you can boast about, but you've got nothing to boast about before God. But He's got everything to boast about before you. Like, "hey, you see this glorious Kingdom? I built all that, and I can let you in even though you don't deserve it, because I deserve it. I built it, it's mine. I bought you, you're mine. See that lake of fire over there? That's what you deserved, but I paid the ultimate price, so you don't have to go there now." Thank God. Thank God that though we don't deserve Him, He loves us enough to restore our relationship anyway.