These verses reveal the humiliation of being a Christian. Naturally, if a man does not hit back, it is because he is a coward; but spiritually if a man does not hit back, it is a manifestation of the Son of God in him. When you are insulted, you must not only not resent it, but make it an occasion to exhibit the Son of God. You cannot imitate the disposition of Jesus; it is either there or it is not. To the saint personal insult becomes the occasion of revealing the incredible sweetness of the Lord Jesus.
The teaching of the Sermon on the Mount is not – Do your duty, but – Do what is not your duty. It is not your duty to go the second mile, to turn the other cheek, but Jesus says if we are His disciples we shall always do these things. There will be no spirit of – "Oh, well, I cannot do any more, I have been so misrepresented and misunderstood." Every time I insist upon my rights, I hurt the Son of God; whereas I can prevent Jesus from being hurt if I take the blow myself. That is the meaning of filling up that which is behind of the afflictions of Christ. The disciple realizes that it is his Lord’s honour that is at stake in his life, not his own honour.
Never look for right in the other man, but never cease to be right yourself. We are always looking for justice; the teaching of the Sermon on the Mount is – Never look for justice, but never cease to give it.