This verse reveals the humiliation of being a Christian. In the natural realm, if a person does not hit back, it is because he is a coward. But in the spiritual realm, it is the very evidence of the Son of God in him if he does not hit back. When you are insulted, you must not only not resent it, but you must make it an opportunity to exhibit the Son of God in your life. And you cannot imitate the nature of Jesus— it is either in you or it is not. A personal insult becomes an opportunity for a saint to reveal the incredible sweetness of the Lord Jesus.
Never look for righteousness in the other person, but never cease to be righteous yourself. We are always looking for justice, yet the essence of the teaching of the Sermon on the Mount is— Never look for justice, but never cease to give it.