He is wise in heart, and mighty in strength: who hath hardened himself against him, and hath prospered?

Wesley's Notes

9:4 He - He is infinitely wise, and searcheth all mens hearts and ways, and discovers a multitude of sins which mens short sighted - eyes cannot see; and therefore can charge them with innumerable evils, where they thought themselves innocent, and sees far more malignity than men could discern in their sins. Mighty - So that whether men contend with God by wisdom or by strength: God will be conqueror. Hardened himself - Obstinately contended with him. The devil promised himself that Job in the day of his affliction, would curse and speak ill of God. But instead of that, he sets himself to honour God, and speak highly of him. As ill pained as he is, and as much as he is taken up with his own miseries, when he has occasion to mention the wisdom and power of God, he forgets his complaints and expatiates with a flood of eloquence on that glorious subject.

Jamieson-Fausset-Brown Bible Commentary

4. wise in heart-in understanding!-and mighty in power! God confounds the ablest arguer by His wisdom, and the mightiest by His power.

hardened himself-or his neck (Pr 29:1); that is, defied God. To prosper, one must fall in with God's arrangements of providence and grace.

Matthew Henry's Concise Commentary

9:1-13 In this answer Job declared that he did not doubt the justice of God, when he denied himself to be a hypocrite; for how should man be just with God? Before him he pleaded guilty of sins more than could be counted; and if God should contend with him in judgment, he could not justify one out of a thousand, of all the thoughts, words, and actions of his life; therefore he deserved worse than all his present sufferings. When Job mentions the wisdom and power of God, he forgets his complaints. We are unfit to judge of God's proceedings, because we know not what he does, or what he designs. God acts with power which no creature can resist. Those who think they have strength enough to help others, will not be able to help themselves against it.