Eternal security is true for those that continue to believe in Christ Jesus but if one that believes chooses to latter reject that faith then they are not saved are they. Those that hold to OSAS would say that no matter what the person would be saved.
But that may not be a possibility. That's what I keep trying to say. I think the reformed explanation of how this works, called the perseverance of the saints, which is slightly different than OSAS is a better explanation. However; being a Baptist, I see how OSAS could be true. And yes, those that hold to OSAS could become complacent or even presumptuous regarding how they live, but they don't always do that.
If the possibility of a believer rejecting Christ did not exist then why the warning passages?
I thought I explained that. It may not make sense for a OSAS person but it makes perfect sense for someone who believes that a saint must indeed persevere, even though doctrinally you know that they indeed will persevere. If you don't think it makes sense then just assume you can lose your salvation. Just be aware that you may have a hard time dealing with other passages that suggest this is not possible.
To think that it is impossible for a true believer to fall away can lead to a false sense of security. Thus the warning passages value.
It could but in all fairness, OSAS believers don't mean that nothing can happen to a person who falls into sin. Sickness, loss, even premature death are often mentioned as well as loss of rewards and the prospect of facing Christ someday as disapproved or rejected for service. There again, if you don't care about that then you probably weren't saved to begin with. If you were then the warnings are still powerful.