Lewis argues that God does not merely want “nice people”, He wants “new men”. He doesn’t want you merely to be a “nice guy” (or gal), He wants to transform you into His son (or daughter). In other words, He wants to change your very nature — from that of a mere creature to that of a son (or daughter) — and not simply improve your personality.
Lewis also talks about an objection that some people have about Christianity: “If Christianity is true, why are some, if not many, Christians mean and corrupt? Why are they no different in their behavior and lifestyle than some non-Christians?” The assumption there is that, if Christianity is true, it should work in that it should make its followers nice and good.
Lewis seems to agree that that is a reasonable objection. He says that if Christianity is true, it should convert the character of the people who profess to follow it.
I’m not sure if I agree. Many Muslims are compassionate. Does it follow therefore that Islam is true? Many Buddhists are peace-loving. Does it follow therefore that Buddhism is true? Many atheists are decent. Does it follow therefore that atheism is true? Obviously not. Similarly, the fact that there are many Christians who are kind and loving does not prove Christianity’s central truth claims that God exists and that He revealed Himself in the person of Jesus Christ (One way of proving the claim that God exists, for example, which Lewis has done, is to show that the existence of a Moral Law points to the existence of a Mind behind the Law). Or, put another way, the fact that there are many Christians who are ill-tempered and obnoxious does not disprove Christianity. All that it shows is that Christians, or people who call themselves Christians, can be hypocritical. But come to think of it, who isn’t a hypocrite? Aren’t we all sinners within the Church? And that is just to be expected, because after all, that is the very reason why Jesus came into this world in the first place — because we are sinners and He wants to save us.
But I agree with Lewis that God does not simply want a surface-level change from us, which is what mere niceness is. It’s very easy to be nice, especially if you are already endowed with a naturally pleasant temperament and you live a comfortable life. It’s much harder if you are naturally irritable and you often experience hardships. What God wants from us, whether we have a natural tendency to be nice or not, is for us to give ourselves to Him and trust in Him so that He may transform our nature into His likeness. It goes without saying that men who are new and transformed also become nice.