I remember the first time I read a summary of the four gospels, back when I was a decade-long and active atheist in my mid-teens. The account was in the preface to the play Androcles and the Lion by George Bernard Shaw (who was also an atheist - albeit writing a play about early Christian martyrs).
I distinctly recall being astonished that this Gospel claimed to be written by one of Jesus's disciples, indeed the disciple who claimed that Jesus most loved him and to whom Jesus gave over the care of his mother. And that none of the other Gospels even claimed to be eye-witness accounts.
I was surprised to find that there was an actual, contemporary, eye-witness account of Jesus, yet it was not (apparently) regarded any differently from the various other Gospels and Books of the New Testament...
Except, implicitly, it was down-graded and put on the average level; or even lower, because the Fourth Gospel was different from the other three, and was therefore repeatedly 'out-voted' and further down-graded when it disagreed.
It was only a few years ago, and some time after I became a Christian, that I reached the conclusion that the whole history of Christianity had been shaped by this decision about how to regard the Fourth Gospel.
A Christianity derived primarily from the Fourth Gospel has many and large differences from one derived from the traditional Orthodox, Roman Catholic, Protestant and other denominations. These all accept the (undisclosed, unexamined) assumptions that all The Bible (or New Testament) should be regarded as equally authoritative and valid (or that in practice the Synoptic Gospels, or particular Pauline Epistles be given primacy to structure the other Books).
Therefore, in practice, the Fourth Gospel has been implicitly regarded as 'nothing special'; Not the primary and best source.
The question here is whether this subordination of the Fourth Gospel was an error or deliberate; and if it was deliberate - to what extent this was 1. Necessary, and/or 2. A Good Thing?
Without getting into historical detail (about which extremely little is known, anyway - and assuming the validity of secular history when applied to scripture is itself another kind of error!) I think it unlikely that the subordination of the Fourth Gospel was an error. I think it was deliberate.
If it occurred to my 14 year old atheist self that surely the Fourth ought to be seen as the most important documentary evidence about Jesus; then I think it would have occurred to the people of the early Christian church who selected and compiled The Bible.
Not least, by placing this first-written Gospel in fourth place; it must have been intended from the start that the Synoptic Gospels should structure our understanding of Jesus's nature, life and mission.
There are several major consequences. Probably the most significant is that it is Matthew and Luke, with Paul, who provide the assumption that Christianity is primarily about a church: an institution; whereas this is contradicted when the Fourth Gospel is regarded as primary. Naturally, the church would notice this, and endorse the sources which validated itself.
A second consequence is the expectation of the second coming of Jesus. This is described in Matthew and Luke (and no whisper of it in Mark or 'John' chapters 1-20, i.e. the original Gospel and the earliest Synoptic); and was apparently of extreme importance to the early church... To the extent that they were prepared to ride-out the apparent anomaly that it had not happened within the (normal, natural) lifespan of the disciples.
A third consequence is the idea of Jesus as a divine being from his birth, and that his birth and early life - as well as death and resurrection - were in fulfillment of Old Testament prophecies. The Fourth Gospel specifically says Jesus was born in Nazareth not Bethlehem, and strongly assumes that he was a 'normal Man' until the baptism by John; whereas Matthew and Luke regard Jesus as a miraculous child, from before conception; whose life ticked-off prophecies all the way through.
My impression is that in order for there to be a Christian Church as an institution, and for that institution to achieve converts from Jews and Pagans and develop a coherent way of life, it was necessary to subordinate the Fourth Gospel.
Was this a good or bad thing? My impression is that it was necessary, and 'therefore' Good; because at stage of the development of human consciousness (which Steiner terms the Intellectual Soul - a phase partway between Original Participation and the current stage of the Consciousness Soul), a church was the only form a religion could take.
The choice those many hundreds of years ago was between Christianity as a church, or not at all. And given that Christianity needed to be a church in order to survive and thrive, that church must be 'about' something; and that 'something' could only be developed by subordinating the Fourth Gospel.
But now, human consciousness has a very different form in the history and destiny of our development. We are in the Consciousness Soul, and need to be aiming at Final Participation.
This explains why institutional churches have weakened and weakened, got further and further from being spiritually Christian; and the past year has seen the greatest, fastest and most profound collapse of the church-based Christian religion since it began.
And this is exactly why the Fourth Gospel has, after nearly 2000 years, come to an acknowledgment of that primacy it always had. Now that churches are either gone or too weak to hold the Christian faith; the individual Christian has become 'Christianity'.
The elements of the Synoptics and Epistles that necessarily dominated church-rooted Christianity have fallen-away; but can, and should, be replaced by a Christianity that takes its lead from the Fourth Gospel: a Romantic Christianity - which was, indeed, in its essence; the original Christianity as taught by the actual Jesus.