Job 9:20

"If I think I have attained purity in my actions, I will be proved perverse in my words. If I am found blameless in my words, I will be reproached for my actions." In the same manner, if one is righteous in his actions and proclaims that loudly with ostentatious words, his mouth commits impiety, because he has fallen into pride, a pride appropriate to the betrayer, the real impious one. If one is blameless but ignores the source of his purity, and as a consequence places his trust in himself and becomes proud and arrogant, he will become perverse. Evidently, the hand of God has abandoned him.

If I desire to justify myself; mine own, mouth shall condemn me; if I say I am perfect, it shall also prove me perverse.

As if it were in plain words; 'Why should I speak about others, when I cannot bear testimony concerning myself?' But whereas thou art not competent to witness to thine own innocency, dost thou know the fact that thou art innocent?

Sometimes, however, although a man has no other witnesses to speak in his behalf, he is still confident in his case because he trusts in the testimony of his own conscience. Yet even the witness of conscience cannot prevail for men against the contrary accusation of God. He shows this in several degrees. The testimony of conscience has three levels, the highest of which is when one's conscience wants to render testimony that he is just, as Romans says, "The spirit himself renders testimony to our spirit that we are sons of God." (8:16) But this witness cannot stand fast against divine censure. He therefore says, "If I should want to justify myself," i.e. if I want to say that I am just, when God instead is objecting that I am unjust, "my own mouth will condemn me," for it will render me worthy of condemnation for blasphemy. The second level is when someone, although he does not presume that he is just, still does not find fault with himself in his conscience for some sin, as 1 Cor. says, "My conscience convicts me of nothing." (4:4) But this witness cannot stand against God either, and so he says, "if I show myself innocent," i.e. if I want to show that I am without sin, "he will prove me wicked," in that he will show sins of which I am not conscious to myself and others. For Psalm 18 says, "Who understands his crimes?" (v.13)