1. I'd like to consider two related objections to the historicity of Scripture.
i) Private conversations
In Biblical narratives we have many instances of what appear to be private conservations. A prima facie objection to the historicity of these conversations is that no witness was present, much less a stenographer, to take down what was said at the time. So how is the narrator privy to that information?
The "skeptical" explanation is that these are fictional conversations which the narrator put on the lips of the characters.
ii) Long speeches
Biblical narratives sometimes contain long speeches. The Sermon on the Mount is a case in point. How could the narrator or his source have verbatim recollection of a long speech he heard just once? People normally remember the gist of what was said.
2. Now let's consider some natural explanations:
i) Private conversations
In some cases, these may not be private conservations. When relaying a conversation, historians typically focus on the principals. That doesn't mean there weren't other people in attendance.
So in some cases, anonymous informants would be available. People in the entourage of the royal court, priestly establishment, and so forth, who are closet Christians, but keep their heads down to avoid having their heads unceremoniously separated from their bodies. Servants and courtiers who privately distain their employers, and are only to happy to leak unflattering information about their employers.
A more specific example might be the Beloved Disciple (John). He normally prefers to remain in the background rather than drawing attention to himself. He only comes forward at strategic points in the narrative to offer his eyewitness confirmation.
There are concentric social circles in the Fourth Gospel. You have an outermost circle of general followers. Then a smaller circle of the Twelve. Then an inner circle of Peter, James, and John. Then the inmost circle of Jesus and the Beloved Disciple. Apparently, John was Christ's most trusted confidant.
So even in scenes where only Jesus and someone else are mentioned, John may be a lurker. He generally maintains a low profile in the narrative to keep the focus on Jesus.
Regarding the Sermon on the Mount, I doubt Jesus said all that at one time. Jesus was an expert communicator, and that's just too much for an audience to absorb in one sitting.
Matthew has a habit of grouping related material. I think Jesus engaged in public teaching on that occasion, and Matthew used that as a hook to combine it with other things Jesus said on other occasions.
An advantage of writing is that you can reread the material. And it's easier to locate the material of it's grouped together by topic.
3. However, there are other cases where natural explanations don't seem to be as plausible. For instance, take conversations involving the patriarchs. There were no witnesses. No transcript which a later writer could consult. Perhaps, though, some of this might be passed down in family lore. Oral history.
Besides the Sermon on the Mount, another example is the farewell discourse, followed by the lengthy prayer of Jesus. That runs roughly from Jn 13:31 through the end of Jn 17. (Scholars disagree on where, exactly, it begins.)
That's a long, dense, dry speech (apart from the true vine parable). Not the kind of thing a listener could normally recall in detail from one hearing.
What about supernatural explanations? Christians can appeal to visionary revelation (which may include auditions), inspired memory, and verbal inspiration. And I think those are viable explanations. Now I'd like to briefly explore a neglected possibility.
According to some conventional definitions, automatic writing is writing produced without conscious intention as if of telepathic or spiritualistic origin, or writing produced by a spiritual, occult, or supernatural agency rather than by the conscious intention of the writer.
Assuming that the record of long speeches and private conversations can't be accounted for by natural means, suppose these are examples of automatic writing, inspired by the Holy Spirit? That wouldn't require the Bible writer to remember or know about the event.
4. Now let's consider some objections to that explanation:
i) It's special pleading. Why not just admit these are fictional speeches?
But is it special pleading? I didn't concoct a novel theory to defend the historicity of Scripture. Automatic writing is a well-documented phenomenon. I'm applying that preexistent phenomenon to these particular examples, as a possible explanation.
ii) Automatic writing is occultic!
It's true that automatic writing is associated with people who dabble in necromancy. However, just because there are ungodly examples of something mean there can't be godly examples of the same thing. The existence of false prophets doesn't taint true prophets. The existence of demonic miracles doesn't taint divine miracles. If lesser spirits can produce automatic writing, surely the Spirit of God is able to produce automatic writing. If evil spirits can produce automatic writing for evil purposes, surely the Holy Spirit can produce automatic writing for holy purposes.
iii) Automatic writing has a naturalistic explanation.
That objection conflicts with (ii). They can't both be right. At least, not across the board.
There's the question of whether "automatic writing" is loosely used to cover disparate phenomena. It's true that depth psychologists may say this is just a case of a human being naturally tapping into his subconscious. And, indeed, that may happen.
But automatic writing often takes place in the context of people who are striving to channel the dead. They endeavor to contact the dead. They open themselves to that influence. They wish to play host to that source.
So it's hardly a stretch to interpret the result as a case of possession by a supernatural agent. That interpretation lies on the face of the phenomenon.
(Which is not to deny that charlatans fake channeling the dead.)
iv) To invoke automatic writing is ad hoc. Where do you draw the line?
As with any explanation, you use it when it's necessary or reasonable to account for something that can't be as easily accounted for by some other explanation.
There are different modes of inspiration. The organic theory of inspiration will suffice for many examples of Scripture. But sometimes direct revelation is required. Sometimes visionary revelation is the source. By the same token, why not automatic writing in some instances?
Take visionary revelation. A seer will experience an altered state of consciousness. But that doesn't mean he always, or even usually, operates in that mindset. He couldn't function if he did. That's just when the Spirit comes upon him.
The Spirit can operate in more subtle and subliminal or more dramatic ways. It ranges along a continuum. At one end, an inspired writer may not be conscious of his inspiration. That's the organic theory of inspiration (e.g. Warfield).
At the other end, consider revelatory dreams and visions, where the Spirit takes possession of the human imagination. In that condition, the human agent is basically a passive recipient.
That would be analogous to the Spirit taking temporary control of a Bible writer to produce a text via automatic writing. That would be a type of verbal inspiration. Verbal inspiration in general doesn't require that. But it's a kind of verbal inspiration.