What sacrifice is sufficient to atone for unbelief? It is my opinion that this is the one unforgiveable sin.
42:6 Therefore, after I abhor myself and (comfort myself) in dust and ashes
Why therefore does he abhor himself and sit in dust in ashes to make himself feel better? Paraphrasing 42:5, "I have heard of you but now I see you." That is, "I've heard tell of You but only now have I 'realized' the truth of Your reputation, Your character, Your integrity, indeed, is unassailable (and I now can't believe that I dared to think any different)."
Matthew 12:32 explains the unforgiveable sin which will not be forgiven either in this age or the age to come to be speaking against the Holy Spirit (the reputation, the integrity, the character of God). So, it seems that sitting in dust and ashes would be insufficient, but Job's gesture had obviously pleased God so much that He restored, and double at that. Why? In retrospect, all atonement has been made in Christ once for all, for every sin other than that of unbelief and, me speaking without actual knowledge until further research on the significance of repenting in dust and ashes, I can only suspect that Job comforting himself in dust and ashes is his acknowledgement that he, Job, is, effectively dead, and trust in God's mercy and resurrective power as the only way 'back' to Him. As we do all know that way, The Way, is provided from the foundation of the world. And that genuine belief is the only sacrifice that is sufficient to offer and, indeed, that can be, as the only such that even have left to. It's all we can offer that wouldn't be a blasphemous (I can't think of an adequate word...insult?) to Christ's offering, and especially now that we have no legitimate excuse that He would not nor has offered it.
When God was asking Job, where were you when... He was affectively informing Job that, if no one else was there, He was there... in answer to Job's complaint that many of us have entertained at least once in our life, "God, where were you when...?" God was there, in Job's suffering, and Job realized that Satan would have used all of his power to destroy Job if it were not for God's hedge around the life of Job in saying, "you may not..."
So, imo, Job's indictment is that he stumbled in faith, which is arguable the most dangerous sin in which there is no atonement for, rather than the 'run of the mill' commandments which all have been atoned for at any rate. And, at the root of unbelief is fear, in Job's case, that God had abandoned him, and his relief comes from the realization that 'I shall never leave you nor forsake you.' And that is the spirit of what Job had spoken rightly, that which is of God's true character.