Luke 20:18

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.)

Everyone who falls on that stone will be broken. Upon whomever it falls, it will grind him to powder. It is one thing to stumble against Christ through evil deeds, another to deny Him through impiety. Whoever is a sinner and yet believes in Him falls indeed upon the stone and is shattered, but is not utterly crushed; for he is preserved by wisdom for salvation. Upon whomever it falls, that is, upon whom the stone itself comes down, and who utterly denies Christ, it will grind him to powder, so that not even a shard remains in which a little water may be drawn. Whether he speaks of those who fall upon Him, who now despise Him or bring Him injuries. Therefore, they do not utterly perish yet, but are nonetheless shattered so that they do not walk rightly. But upon whom it falls, it will come upon them from above in judgment with the punishment of destruction. Therefore, it is said it will grind them to powder, so that the wicked may be like dust which the wind drives away from the face of the earth (Psalm 1).