John Doe, a self-proclaimed christian by what he understands of religion/the bible does something's that self-proclaimed christian Bob Doe doesn't agree with so because of that Bob doesn't see John as a christian and in return John doesn't see Bob as a christian(for not doing what he does)...
Who is correct? Who is the christian?
A more serious answer: they're both wrong in their approach.
Religion is determined by community, not by any religious scripture. Whether a person is a member of a religion comes down to 2 questions:
- do they identify as a member of the religious community?
- are they generally accepted by the rest of the religious community? It's the opinion of the community as a whole that matters, not just the opinion of some individual member.
Scripture only matters in this to the extent that the community decides to use scripture (or some particular interpretation of it, rather) as the basis for their decisions about who to include or exclude.
Edit: complicating things for Christianity is that it's made up of many communities under the umbrella of "Christian."