What he tries to illustrate is that man is a much harsher judge than God is. Since God is all knowing, he will find it harder to make a judgement on an individual because he will have knowledge of both the good and bad sides of one’s character. When someone can see both sides to a story they will be more compassionate and find it harder to reach a firm judgement.
And God doesn’t really live on Earth and the crimes we commit do not affect him directly. They only impact man. Therefore man is a more appropriate judge for other men’s crimes than God is. God with his power serves as a suitable witness as he is able to reveal objectively every detail of a man’s life.