Continuous sin leads to doubt and disbelief. You must have witnessed people who have gone through this? Unfortunately, people who claim to be Christians fall into the slavery of sin - which can lead to disbelief in God and a subsequent falling away from Christ. While I admit that the more "seasoned" Christian is less likely to falter even in difficult times, it does happen. My contention is not that it happens often, but even if it happens ONCE, that destroys OSAS.
Regards
So you're saying that if a person sins, even once, after they claim belief in Christ, then they are no longer saved?
It can't be proven by observation because you can't know for sure if anyone is born again. The only proof is what the bible says about salvation.