A teaching of Jesus' during The Sermon on the Mount.
It is during this teaching that the famous "Turn the other cheek" phrase was spoken. Jesus tells us that if someone hits us, turn the other cheek and let them hit that as well. If someone forces you to do something, do twice as much - put twice as much effort as they require from you.
There are two parts to this - never to take revenge, and give your best, even to your enemy.
When I was first working sysadmin, my manager would never support us in what we did. He would let us firefight problems rather than step back and look at the broader picture. We worked longer and longer hours, until the network crashed so severly that the General Manager demanded that I be moved away from sysadmin into project support.
I Hated my manager - rather than accept that I should have done more to stop the firefighting, I swore revenge on him in my mind. Every time my colleagues discussed any work issues, I would rubbish him. Any way I could make his life harder, I would. The revenge feeling extended to the General Manager as well.
I wouldn't step back - we didn't step back during the sysadmin crisis, nor could I step back during my own crisis.
Now? Now I don't honestly care about them. I know that I should love my enemies, but I think that rather than overcome my anger and revenge feelings, I've just shoved them aside. But I don't want revenge against anyone. Not any longer.