In this world of materialism, opposed to all matter of the spirit and religion; it is significant that so many Christians define their faith materialistically, legalistically, bureaucratically - in terms of checklists, bullet points, terms of service, standard procedures...
Perhaps unsurprisingly, the imagination and individuality gets squeezed-out of Christianity, and some of the most important Christian thinkers are rejected as Christians - both by the anti-Christians (who want to deny Christianity the lustre of their membership) and by Christians themselves (captive to materialism).
Many denominations and churches self-define in legalistic terms; and find themselves permanently at odds withe even very similar denominations and churches - and the whole of Christendom disunited, enfeebled, and pervaded by spite and fear.
Yet these self-styled 'orthodox' Christians, the ones who subscribe to (what they regard as) the objectivity of materialism, those who publicly subscribe to multiple, interlocking and sharply defined creedal statements and elaborate oaths... these have (mostly) become corrupted by leftist politics, and nowadays are By Far the worst of fake Christians - worse than the most notorious unorthodox and heretics of the past.
Why should this be? I think it is a consequence of the corruption of materialism. Materialism was present from the early days after the life of Jesus, as Christianity grew in the context of Judaism and the Roman Empire: both legalistically rooted and permeated. At any rate supposedly-strict legalism and objectivity have failed spectacularly to prevent mass apostasy.
Yet the Fourth Gospel (our most authoritative source) presents the teachings of Jesus in a personal (and very simple) way, as a matter of Jesus's identity and our proper attitude to his person and promise. From this perspective, the public religion of Christianity was misconceived from a very early stage.
If not, then what?
Christianity is, obviously, an internal matter, self-defined. Yet this leaves open the matter of judgement - we must judge others, including whether they are Christian - or in contrast opposed-to and working-against God and creation.
How may this be done without legalism and explicit definition? Well, in exactly the same way that we infer the dispositions, motivations and intentions of other-people about any other important subject. We do it all day and every day. This is the bread and butter of social life, an absolute necessity of social competence.
Of course we do not have any mechanism for imposing our own inferences upon other people. When we judge a person (or institution) to be nasty; this usually cannot be proven to someone who interprets their disposition, motivations and intentions to be nice. Despite this, life goes on.
There is not, and cannot be, a better or more valid definition of Christianity that the serious inference of one individual about another: that is the bottom line. If I am a Christian, then I can and must make a heartfelt judgement about others - insofar as this fact affects me personally.
And I would be a fool to allow this 'intuitive' inference to be overridden by checklists and bureaucratic validations.