The Gospels probably came into being because each successive Gospel writer didn’t like the work of his predecessor. So Matthew probably thought Mark’s Gospel was kind of interesting and a great first go, but he thought he could do better. Why do you write a book unless you think you can do better? Especially if you’re going to incorporate large amounts of that material into your Gospel. And what Matthew does is he actually copies out huge amounts of Mark’s Gospel and then adds some stuff that Mark hasn’t got.
For example, Mark’s not got a birth narrative. You know, he just starts with Jesus as an adult. Matthew says we’ll fix that. We’ll have a better narrative. Same with resurrection appearances. There’s no resurrection appearances in Mark’s Gospel. Matthew says we’ll fix that. We’ll have some resurrection appearances. And I think the same then happens with Luke and with John that they’re trying to improve on the work of their predecessors. And what subsequently happens is other people attempt to write Gospels too, but they’re not actually as strong as works of literature as the four canonical Gospels are. You’re hard put in all the other Gospels that have survived to find any that are as strong in terms of their literary quality as the canonical Gospels in the New Testament.
And ultimately what happened is people just decided that it was a good idea to have four canonical Gospels because then you could say this is the Gospel according to Mark. This is the Gospel according to Matthew. And so they expressed their faith as something that has four different witnesses to the same Jesus, and that’s how they reconciled the idea of having different gospels in the New Testament.