Two quotes to start this off with. First, from atheist Bertrand Russell, answering a question in an essay by the same title, “Why I am not a Christian”: “I think that you must have a certain amount of definite belief before you have a right to call yourself a Christian. The word does not have quite such a full-blooded meaning now as it had in the times of St. Augustine and St. Thomas Aquinas. In those days, if a man said that he was a Christian it was known what he meant. You accepted a whole collection of creeds which were set out with great precision, and every single syllable of those creeds you believed with the whole strength of your convictions. Nowadays it is not quite like that.”
Mr. Russell is about right about this.