20 If I justify myself, mine own mouth shall condemn me: if I say, I am perfect, it shall also prove me perverse.
[AD 450] Hesychius of Jerusalem on Job 9:20-21
“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.

[AD 604] Gregory the Dialogist on Job 9:20
36. 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?