Earlier I suggested that the only way a significant movement to include a new document in the New Testament would get started was if we found something that looked like an authentic, first century apostolic epistle, gospel, or proto-gospel.
Let me clarify what I mean by the latter.
It is standardly assumed that there are lost sources behind the four gospels as we have them. The most talked about is a source called "Q," which is allegedly where a bunch of the material in Matthew and Luke comes from. It is not clear whether Q was an oral source or a written source, but many assume the latter. It also is not clear if Q even existed (there are other ways to account for the material besides positing Q and there are arguments against Q), though this is the standard claim these days. (Personally, I’m not convinced, though I’m open.)
Luke, at any rate, mentions that he used written sources in composing his gospel, and unless he’s referring exclusively to Matthew and Mark, that means there’s a lost source.
If we turned up Q or something else that looked like it might be a source behind the canonical gospels, that would be what I’m calling a "proto-gospel," and it would really set the cat among the pidgeons. The scholarly debates would be endless.
And it would be one of the few things that could conceivably spartk a New Testament inclusion movement.
How would that play out?
First, there’d be a buncha folks going "Ooo! Aaaah!" over the document in an uncritical manner and it would sell a bazillion copies.
Then there’d be a buncha folks going "I’m very favorably impressed, but we mustn’t be hasty."
Then there’s be a buncha folks going "Hey, let’s reserve judgement on this thing."
Then there’d be a buncha folks going, "This looks fake to me."
And finally there’d be a buncha folks going "This new document is of the devil!"
There’d be a big fight that would remain inconclusive for some time, probably generations.
Eventually, some publisher might decide to stick the document in Bibles it’s printing. Then there would be another huge controversy over this. (To mitigate it, the publisher might print the document as an appendix, not claiming it to be authentic or inspired but merely "useful," but that would still start a huge controversy.)
In the end, though, standard Bible would continue to outsell the ones that had the document in it. A few Christians (in newly-created denominations following denominational divides over the new book) might use it, but traditional Christians–who would be and would remain the great majority–would not include it in their Bibles, however fascinated or perplexed they might be by it.
What would the Catholic Church do?
Nothing.
Certainly in the beginning.
In our lifetimes we might get a few cautionary statements, but the attitude of the Church would very much be a "Let’s wait and see" attitude. The Church is not about to preemptorially endorse a work of such a sensitive nature if it might turn out to be fake. Neither is it about to preemptorially condemn such a work if it might turn out to be genuine. We’d get cautionary statements telling Catholics not to regard it as Scripture but to otherwise reserve judgement on it, and that would be about it.
And that’s probably the way it would stay.
Forever.
Hypothetically, the Church could use its infallibility to make a determination that the document falls into one of the following classes:
- Fake
- Authentic but not inspired
- Authentic and inspired but not to be included in the New Testament
- Authenatic and inspired and to be included in the New Testament
But the odds of any such determination at any date, even long after our lifetimes, would be very, very low.
The reason is that not making a determination would be so much easier than making one. It would be hard to prove it fake since, per supposition, we’ve already said that it appears authentic.
It would be hard to prove it authentic but not inspired since (a) we have no independent test for inspiration besides Tradition (which is absent here) and (b) we have no precedent for an authentically apostolic work that is non-inspired.
It would be hard to prove it authentic and inspired but not to be included among the Scriptures because of (a) the lack of a test for inspiration apart from Tradition and (b) we have no precedent for an inspired work that is not to be included in the Scriptures.
It would be hard to prove that it should be put in Scripture because (a) again, no independent tst for inspiration and (b) we have no precedent for including new works in Scripture.
The Church would thus find it much easier to simply downplay the matter, to be open to what value the document might have historically, but not to do anything to encourage folks to think of it on the same plane as the known Scriptures.
The only way I can see an infallible determination being made would be if, probably after centuries, a huge controversy was tearing the Church apart and one was needed for pastoral reasons.
In that case the likelihood would be that the decision would come down this way:
While this document may have many useful and instructive things to tell us, the Holy Spirit did not choose in His providence to shepherd it into the New Testament at the time it was codified. He did not choose to have it be part of the patrimony of Christendom down through the ages. Consequently, since the Scriptures as they have been historically known form the patrimony of the Church that God intended it to have uniquely in all ages of its development, it is hereby infallibly defined that the new document–whatever value it may have–is not to be placed in the canon of Scripture.
Only it’d be said more flowery than that.