If you wanted me to make a complete guess about the dates - I suspect versions of the 4 gospels probably originated from much simpler and shorter oral preaching as "pericopes", designed to instruct the faithful on how they should behave in various situations. That may stretch back to centuries BC! And may have once been similar to what was being preached amongst people in the DSS community of that region.
Whether that evolved into something like early prototype written versions of the 4 gospels, and whether there were at that time numerous other gospels, and what date that might have been, I have no idea.
And I don't think there is anything more anyone can honestly say about it; except that what we now have, and all that remains, as relatively complete and relatively readable extant copies, i.e. the copies that we are actually relying upon for detail (presumably), seem to date from 4th-6th century, and mostly from 6th century onwards.
Textual analysis of the books of the Bible has been going on for several centuries. The scholars have long ago found evidence that biblical texts went through evolution, like how the Pentateuch was put together by combining from several sources and that there was at least three authors for Isaiah. From the New Testament side it was recognized that there was more than one author for Pauline epistles.
Given this large existing body of research, it should be possible to figure out the evolution of the Gospels, shouldn't it?
It turns out that they have uncovered evidence that they included parts of older texts in them. Possible earlier texts include the one called Q, Signs Gospel, and a Passion Narrative. There is no uniform agreement for what these contained or if they all existed, especially Passion Narrative seems to divide opinions. They have also found out that in the late antiquity two different versions of Mark were circulating, longer and shorter ones.
The consensus opinion is that the canonical gospels were written over a short period of time. It took only a couple of decades from writing Mark and Q before they were included in Matthew and Luke and after that the texts stayed essentially unchanged. You propose a much longer period for stabilizing the text. Do you have any evidence for that evolution? Such as linguistic changes in the text that were originally used to identify Deutero-Isaiah? Or a collection manuscripts that are copies from gospels made at different stages of text evolution?
There are the two different Marks, but what about the other gospels? If they went through long evolution, where's the evidence?
We know that there existed a version of Luke that was strikingly different from the current one even though there are no extant manuscripts for it, and you need to be able to explain its existence if you want to propose that Luke was written centuries after Jesus.
The standard version of the story goes like this:
* sometime in the 130s a rich merchant called Marcion converted to Christianity. Around 140 he moved to Rome and became an active member in the church of Rome.
* at some point he became convinced that the teachings of the church to are wrong, and went on to start a reformation.
* he denied the humanity of Jesus altogether and claimed that he was full god all the time.
* Marcion thought that the only reliable gospel was Luke, but that it had been corrupted and he produced a purified version of it by removing all references to the Old Testament and to the humanity of Jesus, such as the birth narrative as a god couldn't be born from a woman.
* in 144 Marcion was kicked out of the church of Rome and he formed his own gaining a following that evolved into the Marcionite sect that survived about 300 years.
* in early 200s Tertullian wrote an account where he enumerated the errors of the Marcian heresy where he went through the changes that Marcion had made to Luke.
If you want to argue that the texts that Tertullian used for the four gospels were not essentially the same as the modern gospels, you have to take the full forgery route where a later writer authored a text in the name of Tertullian where he put him railing against a heretic living 50 years earlier. Gets a bit too conspirational for my tastes, but I'm certain that soon there will be a post here explaining that it's the only possibility. After all, the other thing that Marcion accepted in addition to the purged Luke were the Pauline epistles.
You can also argue that Marcion's Luke was the orginal one if you want. If you do, please give that argument together with some textual analysis that shows that Marcionite and non-Marcionite parts of Luke had a different author. Even if you uncover find the evidence that all other scholars have chosen to ignore, that still doesn't change that Tertullian had essentially the modern Luke. If you deny that, you are back at the full forgery.
But nobody actually cares if it was actually Pythagoras that produced it. Because nobody today is worshipping Pythagoras worldwide in their billions as the son of almighty God!
The existence is of Pythagoras is not important to
you, so you apply one standard for him. The non-existence of Jesus is important to
you, so you apply another. Of course, a Jesus who was 100% human and 0% god is just as damaging to Christianity as a Jesus who is 100% invention so there's no need for the different standards. If your aim is to try to convince billions of Christians or even one singe Christian that Christianity is false, then using double standards might not be the best route.
