I find this situation most difficult. Forms of OSAS are still out there and many subscribe to it wholeheartedly. Like anything else, there are pious people and ones that are not so much. You definitely have to be charitable-- Let’s say that you were one that believed that you are saved because when were younger you told Jesus that “you accept Him as your personal Lord and Savior” so now you’re saved and now your job is to get others to “accept Him too”. The Great Commission, right.
Now, some Catholic comes up and says “hey, you’ve been duped your whole life; you may not “make the cut” because you could be in a state of mortal sin”. How do you react? This is why I say you have to be charitable because you’re delivering a message that threatens their entire perception of faith.
I think a good starting point for you to help organize ideas would be to read Numbers chapters 9-11 and notice how the Israelite people (who were chosen or “saved”) but continued to commit grave sins were punished with death (condemnation). Then read 1 Corinthians 10 and notice how Paul (New Covenant) continually refers to these stories to
make his point to the Corinthians about persevering in the battle unto the end. Notice verse 11 carefully.
Hopefully you can find something in here useful. Good luck and may God bless your efforts.