Why did God need to prove Satan wrong in Job?

Answer 1

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He didn't. God, by definition, knew the faithfulness of Job. The enemy wanted to destroy Job so he could "throw it in the face" of God. When you read Job it becomes apparent that God had blessed Job with family and financially. The enemy said Job was a fake and thought that if he could take these things from Job he would be able to prove that Job was a fake. The enemy was wrong.

Answer 2

The Book of Job follows the modern Jewish notion of Satan as the loyal assistant of God, rather than the Christian notion of Satan as the evil enemy of God. It was Satan's role to prove the righteousness of the faithful.

God believed that Job was a perfect and upright man, the like of whom was no other on earth. Satan answered that in the right conditions every man will curse God, so God gave Satan the challenge to prove this.

After two failed attempts by Satan to have Job curse God, even God became angry and demanded of Job whether he held himself to be equal to God. Both in his challenge to Satan and his later angry monologue to Job (Job 38-41) God seems to be demonstrating that he does not take kindly to those who cross him, no matter how innocently.