Stop me if you’ve heard this before:
The Protestant doctrine of justification by faith alone means they believe they think they can sin as much as they want and still be saved!
When you get that — and you will, if you interact much with Catholics online — just ask if the Bible addresses that possibility. Then listen to the crickets chirping. Then ask them if they’ve ever read Romans.
Romans 6:15 What then? Are we to sin because we are not under law but under grace? By no means!
What?! You mean Paul asked and answered their question? Yes, we’ve had that answer for nearly 2,000 years. If they read the Bible for themselves, they would know it.
Seriously, ask it nicely and try to plant seeds. Hopefully they’ll realize that if they read their Bibles more they’d know the truth.