Did Christians EVER really agree? The first bible only included the Gospel of Luke and 10 of Paul's epistles. The entire Old Testament was tossed.
In early 1st century writing, iirc (we discussed much of this at length in the various Historical Jesus threads here), there were critics of the Jesus stories. That is - various historians of the time wrote to say that people we now call "Christians" were spreading false "gospel", where "gospel" just meant the "news" about true belief in God and the correct religion etc.
However, that is not the same as saying that tens of thousands of Christians all disagreed with one-another over what parts of the OT and what parts of new gospel preaching from Jesus & Paul, should be acclaimed as the truth of God and God's creation of Earth, Universe, and Man.
Over the course of those early centuries when Christianity was forming from the earlier Jewish religion of the OT (and the religious beliefs were all very similar from OT to NT), the faithful were working-out amongst themselves all the details of what they must accept as the holy truth of God.
In any group of tens of thousands of people as it was then (say, circa 400AD), or in the 2.5 billion or so that we have today as "Christians", it's inevitable that you will always be able to find some people who disagree about certain points of a very wide ranging set of claims and beliefs that make up a religious faith. Even leaving religion aside, if you have just 20 people discussing anything at all, then some of them will always disagree on certain specific details.
So it's a total red-herring to criticise any comment or explanation here by saying "but did they all agree?".
The actual question has to be whether or not the leaders of the religious faith developed a set of core beliefs which they themselves professed to believe, and which they decreed to be the beliefs that all people within their faith (ie Christianity in this case), must adhere to in order to be a genuine member of that faith/Christianity.
By the time the early Christian leaders had decided which gospels should be included, and which ones excluded, and hence which claims of God and Jesus were to be accepted as absolute truth, then “yes”, by that time all genuine Christians were supposed to follow that agreed set of faith beliefs.
And those beliefs certainly included all of the claims that we find in the earliest existing copies of that bible. And that includes the belief that early prophets of the OT had actually received the words from God himself. The words of God had told the prophets how God created heaven & earth, how God had created Man etc. And from NT times that also included the words of Jesus and St Paul telling everyone what they must believe as the true words received from God.
That NT “gospel” really did not change much at all from what God himself had told the earlier prophets. It could not change it, because that was believed to be the actual words of God. And that could not be changed … you cannot change what God has said. The only thing that was different by the time of Jesus and Paul, was that all sorts of Jewish preachers had gradually come to believe that a promised day of apocalypse was due at any moment. That was apparently being preached by the group of religious Jews (the “Essenes”) who had written the Dead Sea Scrolls probably beginning as early as 200BC. And that's what Paul was preaching 250 years later by 30AD to 50AD.
But all of that preaching was claimed to have come directly from communication with God. That was the unarguable knowledge of everything given to the faithful by God himself. You cannot argue that God was wrong about the story of complete Creation … ie the creation of everything.
And you certainly cannot argue as a Christian today, that you have the ability to change the words of God or to reject that they ever were the actual words of God. Or more specifically – you cannot do that, and yet still claim that those early prophets received the word of God such that they then knew from direct personal contact with God, what truly was, and is & always will be, the absolutely certain explanation for the existence of the Universe, Earth, Man and all creatures on Earth etc etc. You cannot now change that, because that is the entire basis on which the Christian faith exists …
… if you try to change that, for example by saying that you will “re-interpret” it, then you destroy the basis of the religion … you reject the belief that God ever told any of the prophets any of these things.
So the answer is “Yes”, you cannot later change what was originally claimed as the basis of the faith truly given to the prophets by God himself.
That has to be a truth for all time. Otherwise the religion is shown to be false.
That is really why so little does change over the centuries in any religion, It's because changes like that, concerning the absolute words of God, really must be said to be fixed for all time as unarguable truth.
That's what makes religion very different from any other beliefs or ways that we have of studying and learning about things. That's what makes religion very different from science, or history, or geography or medicine, or any subject that attempts to discover and explain actual truths about the world around us … all those other “ways of knowing” are open to change according to new information that we continually strive to find and improve. But that's where religious belief is completely different – religion is claimed to be known from the word of God … the explanations given by God cannot be questioned or changed.
Nor can you really say that it's posssible either to disagree with the earliest prophets, or to “re-interpret” what they said. That is again disagreeing with God. If you say that the prophets words are ambiguous, then you are claiming that God did not want the prophets to clearly know what he (God) was saying … but in that case there can be no true basis for the religion, because in that case what the prophets told everyone would be wide open to doubt and confusion.