Extra Gospels

There are more than four Gospels, historically speaking. Many more¹. "Gospel" literally translates to "good news", and there were lots of them floating around in the early centuries of Christianity. The ones that made it into the New Testament are considered canon by the major branches of Christianity, but "canon" really only means that they were accepted by the Synod of Hippo in the year 393. Before that, a few of the Apostolic Fathers in the early church had proposed accepted lists of canon works but I'm unaware of any being very influential outside of certain circles. And even after that, the canonical books changed multiple times and today can vary across Christian groups.

There is this idea that the Bible as it exists today fell from the heavens in its current form and never changed. That's simply not true. Catholics, for example, hold the deuterocanonical books to be authentic scripture. Protestants do not. Some books are considered canon by very limited Christian sects, a good example being the Books of Enoch. Churches in Ethiopia consider it canon, but almost no other group does, to my knowledge. What is canon, what is authoritative scripture varies among the thousands and thousands of Christian sects that have come into existence in the last thousand-or-so years. While there are some commonalities among what is considered canon across the Christian religion, there's no single definitive canon that has been accepted by all groups through history. Even the four Gospels that are today almost ubiquitous were not accepted by all early Christian groups.

So there are more than four Gospels. Why weren't they included in the New Testament? The answer is because of lot of them were freaking weird! The Infancy Gospel of Thomas, for example, features a Jesus that kills multiple people for no real reason. They didn't really fit in with the rest of the works. The Synod of Hippo had a few criteria for determining what should be considered canon. The major ones were acceptance by a large contingent of believers and consistency with what were regarded as the correct teachings.² These extra Gospels rarely met the criteria for inclusion, not being widely accepted or not being consistent with any of the rest of scripture.

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1. https://en.wikipedia.org/wiki/New_Testament_apocrypha#Gospels. For a good reference source with bilingual texts, see: Ehrman, Bart and Zlatko Plese, The Aprocryphal Gospels- Texts and Translations, Oxford University Press, 2011

2. Bruce M. Metzger, Michael David Coogan, eds., The Oxford Companion to the Bible contains a more detailed discussion on the idea of canon. Pages 98-104