C.S. Lewis may be a great apologist, but his theology can leave things to be desired. This paragraph from “Mere Christianity” smacks of Universalism.
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 to 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.