The entire point of the Gospel is that we need Jesus to save us because we cannot save ourselves. No amount of good works, nor abstinence from bad works, will ever be pleasing to God because our sin, which 1 John 1:8 says even the best of Christians must admit they have, makes even our righteousness filthy rags (Isaiah 64:6).
This is why it is said that we are saved by grace, through faith, not through works (Ephesians 2:8-9). Nothing we do, and nothing we don't do, is enough to get us saved. But if salvation is by grace when we do good works, or when we don't do bad works, the necessary conclusion is that we can still receive it by grace when we don't do good works, or when we do do bad works. Salvation cannot be by grace if. The moment you add an if, be it "if we do good for the rest of our lives", or "if we don't lose it by some sin", that is definitionally not grace.
In fact, Romans 11:6 explicitly defines grace as the opposite of works. "And if by grace, then it is no longer of works; otherwise grace is no longer grace. But if it is of works, it is no longer grace; otherwise work is no longer work." If even one part of your salvation is by works, be it action or inaction, it is no longer salvation by grace.
My understanding is that Conditional Security is about as close to apostasy as you can get without actually being an unbeliever. I say this not because I am arbitrarily trying to graft in believers in works based Gospels, neither because I am trying to excommunicate people who may actually be saved, but because I can see where one might get the belief in Conditional Security. In theory, the misunderstanding seems to come from where, exactly, salvation begins. To a believer in Conditional Security, the loss of faith would certainly signify the loss of salvation. To a believer in Eternal Security, it would actually be a sign that someone never really was saved to begin with (1 John 2:19). Ultimately, as long as you recognise that it is Jesus' works that get you into Heaven, and not your own, you are saved. John gives us that assurance in 1 John 5:13, where he writes "These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God." (Emphasis mine). Therefore, although Conditional Security is provably wrong, it is not inescapably damning. Nevertheless, it is absurd to conclude that Christ will only cover your sins if you have no sins to cover. If salvation is by grace through faith, the faithful cannot lose it by works.