Albert Barnes 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 lays up his iniquity for his children - Margin, that is, “the punishment of iniquity.” This is a reference evidently to the opinion which “they” had maintained. It may be rendered, “You say that God lays up iniquity,” etc.

They had affirmed that not only did God, as a great law, punish the wicked in this life, but that the consequences of their sins passed over to their posterity; or, if “they” were not punished, yet the calamity would certainly come on their descendants; see Job 18:19-20; Job 20:10, Job 20:28. This is the objection which Job now addresses.

The statement of the objection, it seems to me, continues to Job 21:22, where Job says that no one can teach God knowledge or prescribe to him what he should do, and then goes on to say that the “fact” was far different from what they maintained; that there was no such exact distribution of punishments, but that one died in full strength, and another in the bitterness of his soul, and both lay down in the dust together. This view seems to me to give better sense than any other interpretation which I have seen proposed.

He rewards him, and he will know it - That is, you maintain that God will certainly reward him in this life, and that his dealings with him will so exactly express the divine view of his conduct, that he will certainly know what God thinks of his character. This opinion they had maintained throughout the argument, and this Job constantly questioned.