Charles Ellicott Commentary Job 21:19

"[Ye say], God layeth up his iniquity for his children. Let him recompense it unto himself, that he may know it:" — Job 21:19 (ASV)

God layeth up his iniquity (i.e., the punishment of it) for his children, may be the hypothetical reply of the antagonists in the mouth of Job, and the second clause his own retort: Let him repay it to himself that he may know it.