C. S. Lewis on “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.
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.—C. S. Lewis, Mere Christianity (New York: HarperCollins, 1952), xv.
This idea of “mere Christianity” did not originate with Lewis, but with English Puritan Richard Baxter in the 1600's:
For one sect then to say, ‘Ours is the true Church,’ and another to say, ‘Nay, but ours is the true Church,’ is as mad as to dispute whether your hall, or kitchen, or parlour, or coal-house is your house.
And for one to say, ‘This is the house,’ and another, ‘Nay, but it is that’ when a child can tell them, that the best is but a part, and the house containeth them all.—Richard Baxter, The Practical Works of Richard Baxter, Vol. 1, 53 (paragraphing and punctuation modification mine).