Why trust the Gospel of John?

From Facebook:

Jeff 

What is [the historicity of John] predicated on?

Hays

The eyewitness testimony of the narrator (i.e. apostle John).

Jeff

Same goes for UFOs and bigfoot?

Hays 

From what I've seen, the footage of bigfoot is just a man in a monkey suit. UFOs are a much more complex subject.

Jeff  

I'm just talking about written testimony of those phenomena. Nothing like video.

Hays 

Is your point to dismiss eyewitness testimony out of hand? That leads to extreme and irrational skepticism, including your own firsthand experiences.

Jeff  

I prefer to externalize my knowledge claims, I know my own senses are not always reliable.

You don't have to verify anything about the author's testimony? You take it all of faith?

Hays 

To begin with, there's archeological corroboration for the Forth Gospel (as well as the Synoptics). Peter Williams has a dandy little book on that: Can We Trust the Gospels?

Moreover, there's lots of incidental internal evidence for the Fourth Gospel. It has the hallmarks of oral history, with lots of unnecessary details and digressions. 

Furthermore, there's the argument from undesigned coincidences. 

Finally, there's lots of evidence for modern miracles, which dovetail with the supernaturalism of the Fourth Gospel.

Jeff 

Well, let's say hypothetically John can be boiled down to 1,000 historical claims. What percent of John's historical claims have been verified? It would be cool to have a resource that goes verse by verse, giving the external verification if it exists.

Hays 

That's not a reasonable method of verification. It's not case-by-case verification but whether there's good evidence that the source is reliable. We don't have to revert to amnesia every time we have evidence for a particular claim, starting from scratch with each individual claim, when dealing with the same source of information.

Jeff 

How do you establish good evidence on the reliability of a source except by examining some mystery number of truth claims that source makes? I'm asking about that very criteria that would grant a piece of text handed to me the 'good evidence' label.

Hays 

There's no magic percentage. One basic principle is that if a source is accurate when we happen to have available evidence which corroborates it, then it's likely to be accurate in cases where corroborative evidence hasn't survived. 

But in addition, there are different kinds of internal evidence and a variety of individual touches that indicate firsthand recollection. So it's not reducible to a single formula. I'm mentioning general criteria, not just the Fourth Gospel.