"If Christianity is real, why is there not an obvious difference between the level of niceness of all Christians and the level of niceness of all unbelievers? If there is any truth to Christianity, shouldn't all Christians be noticeably nicer than all non-Christians?"
This is a partly reasonable question to ask, but it is also partly unreasonable as well. It is reasonable to expect that conversion to Christianity ought to make a person kinder, more compassionate, more self-controlled, and all-around better than he was before. If there is no change in the external behavior and the individual goes on just the same as he was before, with maybe only new insights or greater religious interest, it is quite possible that his conversion was largely imaginary. "In that sense, the outer world is quite right to judge Christianity by its results. Christ told us to judge by results. A tree is known by its fruit; or, as we say, the proof of the pudding is in the eating. When we Christians behave badly, or fail to behave well, we are making Christianity unbelievable to the outside world."
(1) The world is not divided up into two distinct camps - 100% Christian and 100% non-Christian. There are many who call themselves Christians and yet are slowly ceasing to live up to the name they claim, just as there are many who don't yet refer to themselves as Christians but are well on their way to belief. Christian and non-Christian are not two mutually exclusive, polar ends of the spectrum, but are rather more like overlapping circles where individuals may be slowly or suddenly turning into one or the other. "When we are comparing Christians in general with non-Christians in general, we are usually not thinking about real people whom we know at all, but only about two vague ideas which we have got from novels and newspapers. If you want to compare the bad Christian and the good Atheist, you must think about two real specimens whom you have actually met."
Really, we shouldn't even be surprised if there are people in the Christian category who are still nasty and not yet nice because it is highly likely that there are more nasty people turning to Christ than there are nice ones. "If you have sound nerves and intelligence and health and popularity and a good upbringing, you are likely to be quite satisfied with your character as it is. 'Why drag God into it?' you may ask. A certain level of good conduct comes fairly easily to you. You are not one of those wretched creatures who are always being tripped up by sex, or dipsomania, or nervousness, or bad temper. Everyone says you are a nice chap and (between ourselves) you agree with them. You are quite likely to believe all this niceness is your own doing: and you may easily not feel the need for any better kind of goodness."
On the other hand, "it is very different for the nasty people - the little, low, timid, warped, thin-blooded, lonely people, or the passionate, sensual, unbalanced people. If they make any attempt at goodness at all, they learn, in double quick time, that they need help. It is Christ or nothing for them. It is taking up the cross and following - or else despair. They are the lost sheep; He came specially to find them."
"If what you want is an argument against Christianity (and I well remember how eagerly I looked for such arguments when I began to be afraid it was true) you can easily find some stupid and unsatisfactory Christian and say, 'So there's your boasted new man! Give me the old kind.' But if once you have begun to see that Christianity is on other grounds probable, you will know in your heart that this is only evading the issue. What can you ever really know of other people's souls - of their temptations, their opportunities, their struggles? One soul in the whole creation you do know: and it is the only one whose fate is placed in your hands. If there is a God, you are, in a sense, alone with Him. You cannot put Him off with speculations about your next door neighbors or memories of what you have read in books. What will all that chatter and hearsay count (will you even be able to remember it?) when the anesthetic fog which we call 'nature' or 'the real world' fades away and the Presence in which you have always stood becomes palpable, immediate, and unavoidable?"