Of course I'd say the idea is false (see James White vs. Trent Horn debate on the topic of "Can a Christian Lose Their Salvation?": https://youtu.be/72TRODe8BdA

But I also say their "assurance" doesn't play out in real life. They say if you fall into serious sin or apostasize, then you weren't really saved to begin with, because a truly saved person wouldn't do those things. The problem is that they know they're *capable* of such sins, so they end up questioning their conversion experience. And even if they THINK they are saved, they admit a people can be deceived about it and shown NOT to be saved by their fruits.

It's as if they're guaranteed salvation no matter what...until shown otherwise -- which is another way of saying they can't have full assurance.

It's more reasonable and biblical and straightforward to say that, after baptism, we are in a saved condition unless we fall into mortal sin. If that happens, then we need sacramental Confession. We only go to hell if we die in a state of mortal sin.

"Once saved always saved" is self-deception and can be very dangerous to one's soul. In fact there have been preachers that said it truly doesn't matter what heinous sins you commit after you're saved -- you can't undo your salvation, they say, though you can miss out on certain heavenly rewards. That's a diabolical teaching.