part 1, part 2, part 3, part 4)
You have heard it said, "An eye for an eye and a tooth for a tooth". But I say to you, Do not resist the one who is evil. But if anyone slaps you on the right cheek, turn to him the other also. And if anyone would sue you and take your tunic, let him have your cloak as well. And if anyone forces you to go one mile, go with him two miles.
When our dignity or reputation is insulted or attacked - like a back-handed slap to the face - we want an eye for an eye! We want to insult the person who insulted us, say hurtful things about the person who hurt us, point out the flaws in the person who pointed out the flaws in us. Instead, Jesus says it's better to just take it without retaliation.
We might like the idea of generosity and, at a push, we might be generous towards those who are grateful but being generous to someone who is suing you and taking some of your stuff? Don't be ridiculous! But that's what Jesus suggests.
In a similar way, we may be happy to sacrifice our time on our terms but Jesus talks about going the extra mile for someone who has forced you to go the original mile! Our default setting is to think "why should I?"
Again, Jesus is exposing that, although our actions may look like we are loving or generous, when we look below the surface we find selfishness in our hearts. Again, he is calling for deeper righteousness than the external show of the Pharisees who may do the right thing but do not show mercy to others others.
How can we possibly begin to do any of these things Jesus says? It starts by understanding the gospel. That Jesus was slapped, beaten and killed in our place. That his dignity was taken, his reputation slandered and his character insulted - falsely. That he was stripped of everything. That he carried his cross and died on it. He turned the other cheek for us. He went the extra mile for us. He took our sin and gave us his righteousness. And we did not deserve it. How can we show mercy to people who (we think) don't deserve it? By recognising we don't deserve it either, but he is merciful towards us.