Vague boundaries

I suppose the gnawing worry that I have is that there seems to be no determinate boundary by which we can clearly say whether one falls under the label 'Christian' or not.  Or at least I cannot think of any necessary and sufficient conditions for what a Christian is.  (It is easier to attempt to do so for being RC or EO, and virtually impossible for being Protestant)

Reading some more of John Wesley recently, I've been wondering whether I've been approaching this in the wrong manner, and I've begun entertaining the possibility that there are no such (non-trivial, finitely long) conditions--that perhaps the boundaries of whether one counts as a Christian are a bit vague.

Socrates (at least as depicted by Plato) reminds us that it is extremely difficult to arrive at definitions of various concepts, nevertheless we can sometimes clearly indicate when something satisfies as being an instant of that concept.  For example, I cannot give you a definition of what virtue is, but I'm quite confident that courage and wisdom are examples of virtue; I cannot provide a definition of a game, but I'm sure that baseball counts as one.  Similarly, I cannot give a definition of what a Christian is (at least, one that is non-trivial and will exclude those who seem as though they should not count as members), but I can provide paradigmatic examples of members:  St. Paul, St. Augustine, St. Thomas Aquinas, Martin Luther, John Calvin, Mother Theresa, Billy Graham, etc.  Of course there will be controversial examples (controversial for some, if not for me), those cases that reside in the "gray", e.g. John Dominic Crossan, Marcus Borg, people who have non-orthodox beliefs but are committed to Jesus (oneness pentacostals, seventh-day adventists), etc.  Moreover, the vague boundaries may be merely epistemic; the boundaries might only appear vague due to our ignorance, and perhaps God is clearly aware as to who is a Christian and who isn't (maybe he possesses some definition of that concept that we cannot conceive due to our cognitive inability).

Perhaps, then, that "mere Christianity" is merely a matter of those who bear a relevant similarity with certain paradigmatic examples of being a Christian.  If this is right, I suspect it will be extremely hard for anyone to determinately call someone else a heretic, which might be for the better--at least from an evangelical protestant point of view.

Not sure.  Just thinking and learning as I go.