"To distinguish between trivial and serious sins is a matter for divine, not human, judgment."

"quae sint autem leuia quae grauia peccata, non humano sed diuino sunt pensanda iudicio."

      St. Augustine, Enchiridion on faith, hope, and charity 21, 78, trans. Bruce Harbert, WSA I/8 (2005), 319-320.  For "We see that some have been pardoned and permitted even by the apostles," while "There are also some . . . that might be thought very trivial were they not shown in the holy scriptures to be more serious than is thought" (21, 79).  Etc.  The point would seem (?) to be that revelation trumps human judgment (and especially the exculpatory judgment that a particular sin is but venial), not that human judgment is wholly incompetent in this regard.  Cf. the citation of this at DTC 12, col. 228.