[T]he situation in the actual world is much more complicated than that. The world does not consist of 100% Christians and 100% non-Christians. There are people (a great many of them) who are slowly ceasing to be Christians but who still call themselves by that name:
some of them are clergymen. There are other people who are slowly becoming Christians though they do not yet call themselves so. There are people who do not accept the full Christian doctrine about Christ but who are so strongly attracted by Him that they are His in a much deeper sense than they themselves understand. There are people in other religions who are being led by God’s secret influence to concentrate on those parts of their religion which are in agreement with Christianity, and who thus belong to Christ without knowing it. For example, a Buddhist of good will may be led to concentrate more and more on the Buddhist teaching about mercy and to leave in the background (though he might still say he believed) the Buddhist teaching on certain other points. Many of the good Pagans long before Christ’s birth may have been in this position. And always, of course, there are a great many people who are just confused in mind and have a lot of inconsistent beliefs all jumbled up together. Consequently, it is not much use trying to make judgments about Christians and non-Christians in the mass. It is some use comparing cats and dogs… in the mass, because there one knows definitely which is which. Also, an animal does not turn (either slowly or suddenly) from a dog into a cat. But 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. Unless we come down to brass tacks in that way, we shall only be wasting time.
Quotes from Mere Christianity, Part 84
C.S. Lewis, Mere Christianity (1952; Harper Collins: 2001) 208-209.