Paul still refers to them as *brothers*.
People fall into sin.
That's what 1 John 1:9 is all about.
David was guilty of adultery and murder and God called him a *man after my own heart*.
Lot was referred to as *righteous* Lot, a term that I'd hardly have thought applied to him considering what he did.
Will believers who sin have to answer for it? Most certainly. Paul attributed the untimely death of some believers on sin in their lives.
Will it cost them their salvation? No. Then it would make salvation based on works.
There's a world of difference between sinning and choosing to reject salvation after one is saved. The first one, everybody does. The second one is up for debate, whether someone who is truly saved can later choose to reject it.
OTOH, there's plenty of speculation as to whether someone who accepts Christ and shows no change whatsoever in their lives was ever really saved in the first place.
Genuine salvation WILL result in a changed heart and life.
That is an undeniable fact. And it will be visible.
I will attest to that. As will those around me who have witnessed the change in me since my Salvation.