A teaching of Jesus' during The Sermon on the Mount.
Jesus reminds us that while murder is a crime and will be punished, so is anger against our neighbour. Jesus says that even if we are in church worshipping and we remember that our neighbour has done wrong against us, we should go straight away to them and make peace.
I find this a hard lesson to practise. It's so easy to hate and be angry towards someone. How many of us can walk up to someone we have wronged and say "Hey, Look, I was wrong to say those things. Can we start afresh?"
I wonder what it is about us that makes it so easy to be angry - often towards people we may never meet, or towards people who really don't mean to hurt us.