Albert Barnes Commentary Job 13:10

"He will surely reprove you If ye do secretly show partiality." — Job 13:10 (ASV)

He will surely reprove you, if you do secretly accept persons — If you show partiality, you will incur his disapproval. This seems to have a very proverbial quality, and to mean that under no possible circumstances was it right to show partiality. No matter for whom it may be done, it will be displeasing to God. Even if it is in favor of the righteous, the widow, the fatherless, or of God himself, if there is not a disposition to judge according to truth and evidence, God will frown upon you. No matter who the parties might be, no matter what their rank, no matter what friendship there might be for one or the other of them, it was never to be assumed that one was right and the other wrong without evidence.

The exact truth was to be sought after, and the judgment made up accordingly. Even when God was one of the parties, the same course was to be pursued. His character was capable of being successfully vindicated, and he would not be pleased to have his cause defended or decided by partiality, or by mere favor. Hence, he encourages people to bring forth their strong reasons, and to adduce all that can be said against his government and laws. See the notes at Isaiah 41:1-21.