This is what Lewis has to say for himself in his introduction:
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. It is more like a hall out of which doors open into several rooms. If I can bring anyone into that hall I shall have done what I attempted. But 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.
Lewis (along with contemporaries such Dorothy L. Sayers and Dietrich Bonhoeffer) played a part in promoting the Ecumenical movement and struggled with the definition of the Church. On the one hand, philosophically there ought to be a unifying system of belief among the body of Christ. On the other, many of us find life in the beliefs and practices that are most divisive. (Depending on which side of the Adriatic Sea you live on, Hesychasm is either "navel-gazing" or experiencing God.)
Mere Christianity proposes a solution that we simply ignore those things we disagree on and talk about those that are common. To use Lewis' analogy of a large building with many rooms, the hallways will have beige carpet and unobjectionable artwork and plants that don't require much in the way of care. In the same way, "mere" Christianity will insist on only those beliefs and practices that remain when the bulk of disagreement is removed.
Shockingly, what's left over turns out to be supremely rich.
After a paragraph in which he explains that some people may spend considerable time in the hallway before finding (what we Protestants sometimes call) a "church home", Lewis address picking a denomination and how we should think of those we disagree with:
In plain language, the question should never be: "Do I like that kind of service?" but "Are these doctrines true: Is holiness here? Does my conscience move me towards this? Is my reluctance to knock at this door due to my pride, or my mere taste, or my personal dislike of this particular door-keeper?"
When you have reached your own room, be kind to those who have chosen different doors and to those who are still in the hall. If they are wrong they need your prayers all the more; and if they are your enemies, then you are under orders to pray for them. That is one of the rules common to the whole house.
More than anything else, Mere Christianity stands as a valiant attempt at ecumenicism. If it fails, it fails because the idea of bringing the entire Christian faith into one house is unworkable. I would prefer to believe that it succeeds.