The horrors of sin in this present evil age have ensured that we've all been wronged, taken advantage of, used, or manipulated by someone else in our lives. The fleshly mind responds vehemently to accusation, seeks retaliation on those who have wronged us, and ensures as many people as possible know that we've been wronged.
But Paul said: "Be kind to one another, tenderhearted, forgiving one another, as God in Christ forgave you." (Ephesians 4:32)
Paul says that being forgiven by God should shape the way that we deal with others who have wronged us. If Jesus, the One who never did anything wrong, endured the cross and grants forgiveness of sins and eternal life those who put their faith in Him, we too should forgive, not hold grudges, and not be bitter.
This is such a simple point, but I shudder at how much my own heart forgets it and seeks to harbor bitterness instead of forgiveness.
Forgiveness is such a profound witness to our faith in Jesus. Pastor John Piper says:
Defining 'saving faith' as the simple belief in your head that God forgave you is wrong. That's not the definition of saving faith. All kinds of people can be deluded into believing that they have been forgiven by God, for all kinds of wrong reasons. What is saving faith then? It is believing that being forgiven by God is an awesome thing. Saving faith is looking the holiness of God, looking at the horror of my sin, and being staggered by the thought that I am forgiven. That is saving faith. Not just the simple conviction in the head that my sins are forgiven because Jesus died for me. You can go right on cruising in your life of unforgiveness and a grudge and justify it by believing you're forgiven, if you're not stunned by being forgiven and you cherish being forgiven.
Faith is a union with Christ that is overwhelmed with the impossible thought that I can be forgiven by a holy God. When you're ravished by it, when you look that forgiveness of God towards you and it staggers you, stuns you, silences you, awes you, and wraps you up, then it becomes a psychological impossibility to turn towards those who have wronged you and say "but I'm not going to pass that on to you". You can't do it. It is a psychological impossibility to cherish the beauty of being forgiven and then not to share it with those who have wronged you.
The battle against bitterness is the battle against unbelief. But belief is not a mere head conviction that Christ forgave you, but a heart conviction that forgiveness is the most awesome thing you've ever experienced.