Claim. Impiety has a single diagnostic: the child does not want to be with the parent, and the parent feels that gap as the absence of true love.
Elaboration. SMM contrasts: “Children of filial piety should not leave their parents, even if a century or a millennium should pass. You should say, ‘I want us to be together!‘… Parental love is such that no matter how bad the children are, the parents think of being with them. This is why it is called true love. What about impious children? They dislike being with their parents” (CSG 14.1 §1, 147-292, 1986.10.1). The biblical antecedent (Cain leaving Abel’s company, prodigal-son patterns) is gestured at — “We find stories of such people in the Bible, do we not?” — and the verdict is that wanting to “live as they pleased” is “the seed of the unfilial.” Filial piety is measured at the level of presence and proximity, not at the level of stated affection.