How do I discuss "once saved, always saved" with Protestants?

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?

The following links should assist you in better understanding the Catholic Church’s teaching on salvation. If you have any further questions or concerns that are not answered by these links, please contact Catholic Answers directly.

Recommended Reading:
Salvation from the Catholic Answers Library
How should a Catholic reply to the question, “have you been saved?”
]* What’s the difference between redemption and salvation?**
]* Where do I find info on salvation?**
]* Why do we hold to conditional salvation?**
]* Confused: I can’t be sure I’m going to heaven?**
]* What about eternal security?**
]* Does 1 John 5 confirm once saved, always saved?**
]* Can one lose salvation?**

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