Bede the Venerable — as excerpted in the Catena Aurea on Luke 20:9-18

“Or else, He who is a sinner, yet believes on Christ, falls indeed upon the stone and is shaken, for he is preserved by penitence unto salvation. But upon whomsoever it shall fall, that is, upon whom the stone itself has come down because he denied it, it shall grind him to powder, so that not even a broken piece of a vessel shall be left, in which may be drunk a little water. Or, He means by those who fall upon Him, such as only despise Him, and therefore do not yet utterly perish, but are shaken violently so that they cannot walk upright. But upon whom it falls, upon them shall He come in judgment with everlasting punishment, therefore shall it grind them to powder, that they may be as the dust which the wind scatters from the face of the earth. (Ps. 1:4.)”