I am in a bible study with some protestants. We are currently studying Romans. We’ve had a fruitful discussion on the once saved, always saved issue. The objection I am getting from a couple people is this: If we are able to lose our salvation to mortal sin, then we are undermining or diminishing what Christ did on the cross and that Christ’s sacrifice is not sufficient, our salvation is now Christ plus our works.
I have attempted to answer this objection by showing various scripture passages to support mortal sin but they seem to think that this is not applying to “true believers” and that the people that is addressed to were never really saved.
How do I best answer this?