In the Sermon on the Mount, Jesus corrected a faulty reading of Leviticus 19:18. “You shall love your neighbor as yourself.” Jewish scribes had added these words to that verse: “and hate your enemy.” But Jesus corrected that tradition by saying, “Love your enemies, bless those who curse you, do good to those who hate you.” 

When God chose to withhold judgment on the city of Nineveh, the demonstrated enemy of Israel, Jonah became angry. He had readily received grace and forgiveness from God for his own disobedience but could not reconcile the idea of God forgiving the Assyrian people. Jonah was violating the eventual New Testament principle of grace: “Be kind to one another, tenderhearted, forgiving one another, even as God in Christ forgave you. Therefore be imitators of God” (Ephesians 4:32–5:1). What God has done for us, we are to do for others.

Is there someone to whom you can extend the same grace that God has extended to you? 

True forgiveness breaks a man, and he must forgive.
Dr. Martyn Lloyd-Jones