The problem is that image of the house with the rooms, illustrating what Lewis meant by “mere Christianity.” It appears in the preface to Mere Christianity.. “I hope no reader will suppose that ‘mere’ Christianity is here put forward as an alternative to the creeds of the existing communions—as if a man could adopt it in preference to Congregationalism or Greek Orthodoxy or anything else,” Lewis writes.No "Mere Christianity", by David Mills. First Things' "On The Square" October 18, 2010.It is more like a hall out of which doors open into several rooms. . . . It is in the rooms, not in the hall, that there are fires and chairs and meals. The hall is a place to wait in, a place from which to try the various doors, not a place to live in. For that purpose the worst of the rooms (whichever that may be) is, I think, preferable.It sounds irenic and ecumenical, but it is a Protestant image for a Protestant doctrine. It makes the Catholic Church a room like any other room. It is a way of saying that the differences between Protestants and Catholics would be solved very easily . . . if Catholics became Protestants.