The title, Mere Christianity, indicates the intention of Lewis, an Anglican, to describe the Christian common-ground. He aims at avoiding controversies to explain those things that have defined Christianity in nearly all places and times. Lewis restates the fundamental teachings of the Christian religion, for the sake of those basically educated as well as the intellectuals of his generation, for whom the jargon of formal, Christian theology does not retain its intended meaning.
Of course, the book has not been received entirely without controversy. For example, the chronicle of Lewis's conversion from atheism contains some of the author's reasons for believing which, as may be expected, some have found to be compelling and others have ridiculed. And both, Christian and non-religious critics suggest that Lewis creates common ground out of beliefs and sentiments that can only be made to appear similar by being purposely vague. In fact, he is explicit in the use of purposeful vagueness at the beginning of the book, when he describes the common ground of all religions, with his point being that Christianity is not mathematics, even if like mathematicans Christians claim that there is only one right answer.
Mere Christianity is widely admired and influential across a spectrum of trinitarian Christians, which may attest to the author's success in accomplishing the aim of restating theology in a way that avoids many controversies.