Historically, there has been a broad consensus about what the doctrinal essentials are. This is illustrated in the way CF handles who is considered Christian; if you adhere to the Nicene Creed, you're in. If you don't, you're not. And it's why, for example, most mainstream churches will re-baptise Mormons or Jehovah's Witnesses but not people baptised in another mainstream church; we don't see Mormons or Jehovah's Witnesses as really Christian, because they don't share that doctrinal core.
However, one of the tensions that has crept in in recent times (and which we see reflected in this thread), is that in response to contemporary social complexity, there's a movement to insist on a set of moral essentials as well. This is a relatively new approach (and a contested one).