I know it is so of a truth - Job here refers, undoubtedly, to something that had been said before; but whether it is to the general strain of remark, or to some particular expression, may be doubted. Rosenmuller supposes that he refers to what was said by Eliphaz in Job 4:17; but it seems more probable that it is to the general position which had been laid down and defended, that God was just and holy, and that his proceedings were marked with equity. Job admits this, and proceeds to show that it was a truth quite as familiar to him as it was to them. The object of his dwelling on it seems to be to show them that it was no new thing to him, and that he had some views on that important subject which were well worthy of attention.
But how should man be just with God? - Margin, "before." The meaning is, that he could not be regarded as perfectly holy in the sight of God; or that so holy and pure a being as God must see that man was a sinner, and regard him as such; see the sentiment explained in the notes at Job 4:17. The question here asked is, in itself, the most important ever propounded by man - "How shall sinful man be regarded and treated as righteous by his Maker?" This has been the great inquiry which has always been before the human mind. Man is conscious that he is a sinner. He feels that he must be regarded as such by God. Yet his happiness here and hereafter, his peace and all his hope, depend on his being treated as if he were righteous, or regarded as just before God. This inquiry has led to all forms of religion among people; to all the penances and sacrifices of different systems; to all the efforts which have been made to devise some system that shall make it proper for God to treat people as righteous.
The question has never been satisfactorily answered except in the Christian revelation, where a plan is disclosed by which God "may be just, and yet the justifier of him that believeth." Through the infinite merits of the Redeemer, man, though conscious that he is personally a sinner, may be treated as if he had never sinned; though feeling that he is guilty, he may consistently be forever treated as if he were just. The question asked by Job implies that such is the evidence and the extent of human guilt, that man can never justify himself. This is clear and indisputable. Man cannot justify himself by the deeds of the law. Justification, as a work of law, is this: A man is charged, for example, with the crime of murder. He sets up in defense that he did not kill, or that if he tools life it was in self-defense, and that he had a right to do it. Unless the fact of killing be proved, and it be shown that he had no right to do in the case as he has done, he cannot be condemned, and the law acquits him. It has no charge against him, and he is just or justified in the sight of the law. But in this sense man can never be just before God. He can neither show that the things charged on him by his Maker were not done, or that being done, he had a right to do them; and being unable to do this, he must be held to be guilty. He can never be justified therefore by the law, and it is only by that system which God has revealed in the gospel, where a conscious sinner may be treated as if he were righteous through the merits of another, that a man can ever be regarded as just before God; see Romans 1:17, note; Romans 3:24-25, note.

I know it is so of a truth - I acknowledge the general truth of the maxims you have advanced. God will not ultimately punish a righteous person, nor shall the wicked finally triumph; and though righteous before man, and truly sincere in my piety, yet I know, when compared with the immaculate holiness of God, all my righteousness is nothing.

I know [it is] so of a truth: but how should man be (a) just with God?
(a) Job here answers Eliphaz and Bildad's oration, touching the justice of God, and his innocency, confessing God to be infinite in justice and man to be nothing in respect.

I know it is so of a truth,.... That is, that God is just, and does not pervert justice and judgment, as Bildad had observed, Job 8:3; Job was a man of great natural parts and capacity; he had a large share of knowledge of things, natural, civil, and moral; and he was a good man, in whom the true light of grace shined; and being, enlightened by the spirit of wisdom and revelation, in the knowledge of divine things, he knew much of God, of his being and perfections, and of the methods of his grace, especially in the justification of men, as appears by various passages in this chapter; he knew that God was just and holy in all his ways and works, whether of providence or grace; and this he kept in sight amidst all his afflictions, and was ready to acknowledge it: he knew this "of a truth"; that is, most certainly; for there are some truths that are so plain and evident that a man may be assured of, and this was such an one with Job; he had no need to be instructed in this article; he was as knowing in this point, as well as in others, as Bildad or any of his friends; nor did he need to be sent to the ancients to inquire of them, or to prepare himself for the search of the fathers, in order to acquire the knowledge of this, to which Bildad had advised; yet, though this was so clear a point, about which there was no room for further contest; but then the matter is:
how should man be just with God? if not angels, if not man in his best estate, in which he was vanity when compared with God; then much less frail, feeble, mortal, sinful men, even the best of men, considered in themselves, and with respect to their own righteousness: for, to "be just" is not to be so through an infusion of righteousness and holiness into men, which in the best of men is their sanctification and not their justification; but this is a legal term, and stands opposed to condemnation, and signifies a man's being condemned and pronounced righteous in a judiciary way; so a man cannot be adjudged, reckoned, or accounted by God upon the foot of works of righteousness done by him; since his best works are imperfect, not answerable to the law, but very defective, and so not justifying; are opposite to the grace of God, by which, in an evangelic sense, men are justified; these would encourage boasting, which is excluded in God's way of justifying sinners; and could justification be by them, the death of Christ would be in vain, and there would have been no need of him and his justifying righteousness: especially, it is a certain thing, that a man can never be "just", or "justified with God", in such a way, or through any righteousness wrought out by him; that is, either he is not and cannot be just in comparison of God; for, if the inhabitants of the heavens are not pure in his sight, the holy angels; and if man, at his best estate, was altogether vanity when compared with him, what must sinful mortals be? or not be just at his bar; should he mark their iniquities, enter into judgment with them, or an action against them, summon them before him to answer to charges he has to exhibit; they could not stand before him, or go off acquitted or discharged: or in his account; for his judgment is according to truth; he can never reckon that a perfect righteousness which is an imperfect one: or in his sight; for, though men may be just in comparison of others, or at an human bar, in an human court of judicature, and in the account of men, and in their sight, to whom they may appear outwardly righteous, as well as in their own sight; yet not in the sight of God, who sees all things, the heart and all in it, every action, and the spring of it; see Psalm 143:2 Romans 3:20; in this sense, a man can only be just with God through the imputation of the righteousness of Christ, accounting that to him, putting it upon him, and clothing him with it, and so reckoning and pronouncing him righteous through it; and which is entirely consistent with the justice of God, since by it the law is fulfilled, magnified, and made honourable, and justice satisfied; so that God is just, while he is the justifier of him that believes in Jesus, Romans 3:26.

REPLY OF JOB TO BILDAD. (Job 9:1-35)
I know it is so of a truth--that God does not "pervert justice" (Job 8:3). But (even though I be sure of being in the right) how can a mere man assert his right--(be just) with God. The Gospel answers (Romans 3:26).

I know - That God is just in all his ways, that he doth ordinarily bless the righteous, and punish the wicked. Before God - And I know that no man is absolutely just, if God be severe to mark what is amiss in him.