Unbelievers are skeptical about the Gospels. That's a self-defeating skepticism on their part, because it commits them to general skepticism regarding testimonial evidence, yet they themselves rely on testimony evidence for most of what they believe.
1. However, I'd like to consider a limiting case. Take dreams. At best, dreams are at least one step removed from reality. Indeed, we usually classify what we experience in dreams to be a paradigm case of something imaginary–in contrast to what we experience when we're awake. Philosophers use dreams as paradigm-examples of illusion. Some researchers classify dreams as hallucinations.
Suppose a biographer's only source of information about the subject was his dreams. Suppose a biographer had direct access to the subject's dreams. The biographer could see what the dreamer was dealt. How much could a biographer reconstruct about the subject's actual background from his dreams? That doesn't seem like very promising raw material.
Perhaps the least reliable part of dreaming is the plot. The plot is imaginary. Even if, in a sense, you dream about what happened to you that day, when you were awake, the overall dream plot will deviate significantly from what really happened.
Dreams have two other unrealistic features. We dream about imaginary characters. Strangers. People we never met in real life. And we dream about them just once.
Likewise, we dream about imaginary places. Strange, sometimes surreal landscapes we've never seen in real life.
However, dreams also have features that correspond to real life. Sometimes we dream about real people. Acquaintances. Usually family and friends–or coworkers. When we dream, we recognize certain people–unlike strangers we encounter in dreams.
Likewise, sometimes we dream about familiar places. Where we live and work, or used to live and work.
In my observation, recurring dream characters are based on real people. Likewise, recurring dreamscapes are based on real places. And when we dream about familiar places, these can be detailed and fairly accurate.
If all I knew about you was your dreams, one way I could sift the core biographical elements from the imaginary elements is by distinguishing the recurring characters and recurring dreamscapes from one-off encounters and one-off dreamscapes.
A biographer could figure out the time period in which you lived from the cityscape in your dreams. If it's a 20C cityscape rather than a 19C cityscape or 18C cityscape or medieval cityscape or ancient Near Eastern cityscape. He could draw the same inference from the way people dress. And the cars. Or furniture in houses. Interiors as well as exteriors. So he could place you within a particular period in history. This is true even when you dream about strange places you've never seen in real life. For imaginary scenes will still reflect your generic experience of architecture from your own time and place.
By the same token, if you dream about high school on a regular basis, he could reasonably infer that you're a teenager. He could infer that from the setting, and classmates–if they're recurring characters.
He could infer your nationality from the language you use other dream characters use. He might well be able to infer your social class from the dream characters you hang out with.
If you have erotic dreams, he could infer if you're heterosexual or homosexual.
From recurring dreams and nightmares, he might be able to infer your unrequited yearnings and deepest anxieties.
If the dreamer is religious, that will sometimes be reflected in his dreams.
2. Let's consider another limiting case. Take forgeries. In the nature of the case, a forgery stands in contrast to history or reality. Typically, a forger impersonates an eyewitness about a time and place other than his own. What makes it detectably a forgery is the telltale presence of anachronisms. That's because the forgery knows his own period better than the period he feigns. Indeed, he's so conditioned by his own period that he can't put enough conscious distance between himself and his impersonation to be aware of the anachronisms.
And therein lies a paradox. Although a forgery is an unreliable or worthless window into the fictitious past setting, it can be quite informative about the forger's background and interests. The Koran's garbled versions of OT events and the life of Christ are historically worthless. However, the Koran is highly revealing about Muhammad's time, place, character, loves, hates, foes, and followers. Likewise, although the Mormon "scriptures" are historically worthless in reference to the fictional past they clumsily portray, they unwittingly reveal a lot about Joseph Smith's character, interests, and the religious currents of the day. Same thing with apocryphal Gospels. Paradoxically, even an unreliable source can be indirectly reliable in terms of what it unintentionally divulges about the circumstances and agenda of the author. They tell you nothing about the projected situation, but quite a lot about the situation of the forger.
My point is to mount an a fortiori argument: if it's possible to learn a lot about a person from his dreams, or forgeries, surely it's possible to learn a lot about a person from historical sources, even if those are generally unreliable.