Eternal Security

But that's the point of the argument, I believe.

A "child of God" who willingly sins, and refuses to repent, MUST question whether they're truly saved to begin with.

Me, I'm not gonna question someone else's salvation; but as Jesus said, we shall know them by their fruits.

 

Of course, a child of God will question their salvation at that point, they will lose all confidence in their own salvation. I would lose confidence in their salvation as well, but, like you said we can't judge whether or not the person is actually saved or not, that is God's perogative. We can only make a reasonable evaluation (by their fruits). We can't be 100% sure one way or the other.