This passage is thick with theology, speaking of justification by faith alone and Christ's role as mediator between God and man. These doctrines are dealt with indirectly in this chapter; we do not see the full revelation of them as in the New Testament, but we see how necessary they are.
"How Can a Man Be Righteous Before God?" This is the most fundamental question that a man can ask. Job's friends assume that it can be done through personal effort and good works, but in comparison to God's holiness and transcendence, man has no standing, no case to argue, and no grounds to even demand a hearing. Even the "righteous" need mercy. This predicament is answered by the work of Christ and the imputation of His righteousness to us by faith.
"He Destroys the Blameless" While being more perceptive than his friends on this matter, Job in his passion takes the point too far by suggesting that God is morally apathetic. At the very least, Job does not retreat from the hard fact that God is sovereign in all things, including human hardships. "If it is not He, who else could it be?"
"Why Then Do I Labor in Vain?" Just as denial is no solution to hardship, self-effort is no solution to our spiritual condition.
"Nor Is There Any Mediator Between Us" Job sees the need for a mediator. This is the role that Christ came to fulfill for His people. Through Christ we have gained access by faith into the grace in which we now stand, and we may approach God to receive mercy.