I believe C. Lewis wrote about it in his book Mere Christianity, saying that Christianity isn't about becoming "nice people" but rather "new men".
Have you ever heard an old saying: "It's better to be kind than to be right" ? It sounds plausible enough, but is it really? Or rather, can something which is not right, be kind? Is it a kind thing really to assist an addict in his addiction, for instance? Is it a kind thing to refuse to confront a sinner in his sin?
Imagine if the society as a whole acted on this principle. We wouldn't be able to punish criminals because sending someone to prison isn't a kind thing to do. Police wouldn't be able to stop a terrorist because shooting someone is not nice.
Being a Christian is not about feeling a certain way, it's about doing the right thing, whatever the cost. Sometimes it can even mean disturbing the peace ( Think not that I am come to send peace on earth: I came not to send peace, but a sword.) Sometimes it means saying unpleasant things to those you love, to prevent them from making mistakes. Sometimes it means that people won't perceive you as "nice". I doubt Jesus thought about being "nice" when he kicked the moneychangers out of the Temple. John the Baptist wasn't being nice, either, when he confronted Herod about his sexual escapades.
May be I'm just being spergy, but I don't understand how people can call themselves Christians and then turn around and in live in the same manner as unbelievers and even worse. The only sin left is "hurting someone's feelings." I think this comedy sketch I posted illustrates this approach perfectly. You couldn't run a kindergarten in this manner, let alone a country. May be, that's why our society is such a mess?