Conditional Security, the doctrine that posits man can lose salvation through sin, is the literal antithesis of the Gospel. Much like any other works based Gospel, it produces pride in the believers who think they are doing well enough to be saved, and terror in believers who believe they have lost, or may lose their salvation. In both scenarios, a weak view of how bad sin is and how good Jesus is are also produced.
I remember a conversation I had with two believers in Conditional Security that demonstrates this point very well. I had posed my belief that repentance is a permanent thing. It's not a case of sinning, saying sorry, sinning, saying sorry, and repeating that process until you either die in sin and go to Hell or get lucky enough to die in repentance and go to Heaven. Rather, it is my view that repentance is about being done with sin, even if sin is not done with you. It is an ongoing war against sin, which will inevitably continue to plague the believer, but we continue to be shaped into Christ's image over time.
The two disagreed, and so I asked them if they were sinners. Both of them denied it. "We used to be", they claimed, but now they're not. So I showed them 1 John 1:8 and asked them again: Are you sinners? Again, they denied it. So I read them the verse again, and a third time I asked them "are you sinners?" A third denial. They just weren't getting it. John tells us that if we say we have no sin, we are deceiving ourselves and the truth is not in us. Well, here these two men were, saying they have no sin, and so the word of God says they are deceiving themselves, and the truth is not in them.
So, we see the pride in this kind of person. "I have no sin, I'm good enough for Heaven." No, you are categorically not good enough for Heaven. You are a filthy sinner, deserving of nothing but wrath from the Lord, and it is entirely because of His great love that He made Christ become sin for you. Every single human being that has ever, or will ever go to Heaven, is there because Jesus Christ took the wrath of God they deserved.
But what if they'd answered correctly? If these men had answered "yes, we are sinners", they would also have refuted the doctrine of Conditional Security. Why? Because the wages of sin is death. God does not distinguish between "big" sins and "small" sins in the way we do. As James said, if you keep the whole law, but offend just once, you are guilty of all (James 2:10). This is because of the Holiness of God. The same God who commanded against murder commanded against adultery, theft, covetousness, lust, greed, dishonesty, it's all one God who commanded these things, and thus it is all one law.
So, where's the line? If all sin is grievous enough to get us sent to Hell before the cross, does it not make sense that all sin is just as grievous after the cross? What sense does it make, then, for the cross to be strong enough to cover some sins, but too weak to cover others? It makes no sense! The logical conclusion, therefore, is that if maintaining our salvation requires us to avoid sin, and yet we are all fully aware of the fact that we cannot avoid sin, that the cross us futile! No one can be saved! The cross was just a nice gesture.
I'm pleased to report that this is not the God we serve. John continues his epistle by saying "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." (1 John 2:1). Christ is our advocate. Christ ensures we do not lose our salvation through sin, be it "great" or "small". You sin, you're worthy of wrath. You have faith, you won't receive that wrath, because the only human being to have ever walked the earth with a legitimate claim to be sinless has received that wrath on your behalf.
As much as believers in Conditional Security claim this is just an excuse to sin, it really isn't. As Christians, we ought to know what a plague sin is, and avoid it as such, not because we're scared of Hell, but because it is right. It is good to avoid sin. It is good to serve God, regardless of how He acts in response. Even if God was to guarantee you would actually go to Hell, it would still be good to serve Him in earnest. Why? Because reward and punishment are inconsequential in and of themselves. As it is, Jesus guarantees Heaven to all who believe in Him (1 John 5:13). Should we not act as if we have this guarantee? Those in Heaven already see proof of this guarantee. They're there, they're not getting kicked out. Is Heaven a Kingdom of debauchery? No, it's a Kingdom of Holiness, because it is God's Kingdom, and so if we believe we already have citizenship there (Philippians 3:20), should we not also act like it?
Possible interjection: This is contradictory. One way, Brian says reward/punishment are inconsequential. The other way, Brian suggests the reward should motivate us.
Answer: Although it seems contradictory at first, in the former case, I am talking about morality for morality's sake. In the latter, I am saying that although God could condemn us regardless, the fact that He shows us such great mercy only highlights His goodness, which should motivate us even more to give Him His glory, as we are recipients of it. In both cases, the right thing to do is obey Him to the best of our abilities, but in the latter case, we are recipients of extra grace (extra grace being a redundancy, as grace is, by definition, extra). To illustrate this further, consider that demons do not receive this grace, and yet they still have moral obligations. By sinning further, they are not only storing up extra wrath for themselves, but are, indeed, sinning, which is wrong. God is merely responding accordingly to their wrongdoing. In our case, God responded to Jesus according to our wrong doing.
Conditional Security, far from being a Holy doctrine springing from the Bible, is actually a pride filled doctrine that downplays the severity of sin, emphasises works in salvation, and undermines the grace that we, by faith, receive from Christ. If you believe you are a sinner saved by the grace of Christ, the conclusion that you can lose that salvation through bad works is just one more bad doctrine you need to repent of.