Albert Barnes Commentary Job 9:3

"If he be pleased to contend with him, He cannot answer him one of a thousand." — Job 9:3 (ASV)

If he will contend with him - That is, if God enters into a controversy with man. If God chooses to charge him with a crime and to hold him responsible for his deeds. The language here is taken from courts of justice, and means that if a trial were instituted, where God should submit charges and the matter were left to adjudication, man could not answer the charges against him .

He cannot answer him one of a thousand - For one of a thousand of the sins charged against him. The word “thousand” here is used to denote the largest number, or all. A man who could not answer for one charge brought against him out of a thousand must be held to be guilty; and the expression here is equivalent to saying that he could not answer him at all.

It may also be implied that God has many charges against man. His sins are to be reckoned by thousands. They are numerous as his years, his months, his weeks, his days, his hours, his moments; numerous as his privileges, his deeds, and his thoughts. For not one of those sins can he answer. He can give no satisfactory account before an impartial tribunal for any of them. If so, how deeply guilty is man before God!

How glorious that plan of justification by which he can be freed from this long list of offences, and treated as though he had not sinned!