My understanding of the Fourth Gospel, and from it the intentions of Jesus with respect to his 'message', is that Jesus provided for the teaching of Men by the Holy Ghost, rather than by Men.
I do not think that Jesus intended Christians to be institutionally organised (like a church) but instead to grow, person by person, loving family members - on the model of the disciples. Presumably many such families would develop, budding-off from the disciples.
What then of teaching about Jesus, and his message? What about scriptures? Well, the life of Jesus and existence of the Fourth Gospel itself implies that there was envisaged a helpful role for teaching.
But I notice that the content of this teaching (by Jesus, and by the author of the Fourth Gospel) was very simple - consisting mostly of different ways to express the two truths of Jesus's divinity, and his offer of everlasting, resurrected Life.
Such teaching would, presumably, be helpful in clarifying what was needed; but so few and such simple truths ought to be discoverable by each Man, from direct intuition: that is, from the direct teaching of the Holy Ghost.
Explicit 'external' teaching might speed-up the process - but on the other hand, might inculcate the bad habit and potentially false practice of looking to Men for answers, rather than to the Holy Ghost. The needs of teachers might over-elaborate, and when the teachings had become high volume, it would be easily distorted, difficult to retain the proper focus.
In fact, Christianity (apparently) took a very different path from that envisaged by Jesus in the Fourth Gospel. It became identified as an organised, institutional, international church (then, soon, multiple churches) - broadly like the many other churches of contemporary Jewish and Roman society. Its teaching became massively elaborated and systematised into a prescriptive way-of-life.
And of course, ultimate authority was displaced from the direct apprehension of the Holy Ghost to an ideal of obedience to an ordained priesthood.
If we take the Fourth Gospel as our ideal, it is hard to know what - by comparison - the overall effect of the various Christian churches has been. At times the core teachings seem to have been reversed. For example, Jesus's offer of Life Everlasting on condition of faith in him; sometimes seems, in practice, to have been inverted into a threat of torment everlasting unless a pattern of prescribed behaviours are followed.
The loving and small scale world of Jesus's family of relatives and disciples, as described in the Fourth Gospel, has given way to hierarchical bureaucracies - regulated by abstract laws, elaborate rituals and procedures, and differentiated by formal training and certification processes.
The consequence has often been Christianity as a mechanism for Good Behaviour, to the extent that the original simple teaching has been all but lost, even when and where Christian Churches were dominant and popular.
Indeed, I wonder if people would have been overall better-off with no teaching at all, and no external knowledge; than with the vast, complex, internally-contradictory mass of supposedly-Christian teachings that has been the actual experience for most Christians since the death of Jesus?
Of course, a great deal of worldly benefit would then have been lost; but perhaps a simplicity, clarity and directness of understanding would have more than made up for it?
But what would 'it' have been like?
Well, presumably there would have been other religions dominant. These would have set the social frame, the external system.
Most people would never have heard of Jesus, and there would be no possibility of 'Christianity'. Instead, the teachings of the Holy Ghost would have been a secret hope, mostly a private experience of the heart.
Jesus would have had another name, or no name; the possibility of resurrection, life eternal, becoming Sons and Daughters of God would perhaps have lacked articulation and precision; but these would have been experienced simply as an inner of experience of contradiction to the 'official doctrines' of other religions.
I think the phrase secret hope seems to catch the experience quite well. People would have been taught despair (in various forms) yet 'inside' they would have experienced hope, assurance, confidence, joy.
Their deepest and most convincing experiences would have been this 'faith' - and the experience would have been one of direct contact with another person; experience of the love of that person and the confidence to believe and follow.
Such secret hope might well have been communicated, personally and in private, among family members, husbands and wives, best friends - but probably, shared (if at all) only among a circle of trusted people; because of its contradiction with the local official religion.
There could not really have been any arguments or evidence to support the secret hope - since there was no church, no theology, no scriptures... There could only be an appeal to the most fundamental personal conviction based on those moments when we each feel most deeply in-touch-with reality.
Nobody would know the extent of this secret hope. On the one hand, a believer would either be alone or supported by very few people; on the other hand, exactly because of this, the secret hope would be inextinguishable.
