I teach a course at the university of Guelph/Humber called informatively 'Judaism, Christianity,Islam'. Recently I ended the component on Christianity rather unsatisfactorally. On the drive home that evening I thought about how I should and could have summarized my lectures on christian history. The thought occurred to me that there did not seem to be one religion, but rather many operating under a very loose label called Christianity.
Does anything tie the ultra Calvinist 'everyone but us is going to Hell' churches (plural even here), with the Anglican 'I will give communion to your dog' church?
I suppose a belief in Christ - but none of these many thousands of separate organizations seem to view, or understand Christ in the same way. There is an old saying among Jews that a room with seven Jews will produce eight opinions - but hard history has kept a sense of 'nation' alive in a real sense. Muslims have the Qur'an which is authoritative only in classical Arabic to centre their Ummah or sense of oneness in community. But Christians have thousands of translations of the Bible in thousands of languages. Christians of the main churches also venerate two thousand years of differing interpretations of the main language translations. Orthodox and Catholic Christians also venerate 'Holy Tradition' - the guidance of the third person of the Trinity.
I will leave this here as I certainly do not have the answer, but would love comments!