"There is and always has been the Church, and various heresies proceeding from a rejection of some of the Church's doctrines by men who still desire to retain the rest of her teaching and morals. But there never has been and never can be or will be a general Christian religion professed by men who all accept some central important doctrines, while agreeing to differ about others. There has always been from the beginning and will always be the Church, and sundry heresies either doomed to decay, or, like Mohammedanism, to grow into a separate religion. Of a common Christianity there never has been and never can be a definition, for it has never existed." --Hillaire Belloc
An interesting point, and one that is followed up at realclearreligion.com.