Indirect/Circumstantial evidence is a thing. A valid thing.
If you watch your father rob a store you have directly observed. If you see a man with a mask on wearing your fathers shirt and watch, holding your father’s gun, about the size of your father, barking orders in the voice of your father, running off with a slight characteristic limp that your father has and driving off in your father’s car while blasting your father’s favorite rock song “Danger Zone,” you have indirectly observed. Is it any less valid? No.
Indirect evidence is used in law and also in science. Atoms, climate change, dark matter, evolution and pretty much all knowledge ever involves some degree of indirect evidence.
In some cases indirect evidence is superior to direct evidence. When we directly observe data it is only as valid as our ability to accurately attain, process, store and recall such data. Humans are notoriously bad at all of these.
All the evidence in the world can’t prove anything. If every apple I ever observe is red does that imply that green apples cannot or do not exist? Of course not! It simply gives evidence toward the claim that green apples cannot or do not exist. It doesn’t “prove” in the sense of certainty, it simply makes it more reasonable than the competition. The claim itself in turn can become indirect evidence.
A book contains a series of claims. If a book says green apples do not exist then the book is more valid for the verification of its claim. The book is not “proven” in the sense of certainty, but it is deemed more reasonable than, say, a book which claims that some apples are green (possibly true but not observed to be true), and far more reasonable than the book which claims that all apples are green or there are no red apples (demonstrably false).
Enter the other important concept: Unfalsifiability. If a book makes an unfalsifiable claim does it work against its validity? Certainly not! Neither does it work in its favor. Unfalsifiable claims, for the rational mind, are truth-neutral. When presented a single unfalsifiable claim, ceteris paribus, the rational mind would be agnostic.
Two other factors now play in: Trust and deception. In statistics as well as other sciences as a common practice we predict based on observed data. This means if a book has a series of claims and the first one is true we will assume the others will be true until one is shown to be wrong. When that one is shown to be wrong we adjust our expectations such that the rate of truth is considered = (True/Total). With regard to the Bible, in the words of Frank Turek, “If the first verse is true, every other verse is at least believable.”
This “rate of truth” has long been termed more simply “trust.” If someone says they will do something and they do it this builds their trustworthiness/honesty/integrity. If someone tells you many things you continually find to be true they earn your trust. All of these things mean that when they make a claim you will be more willing to believe it. This is a completely rational process. As people say things that are demonstrably false they become less trustworthy.
Given that something is unfalsifiable, but is claimed as true from a trusted source, it is reasonable to believe the claim as true because the source itself is operating as indirect evidence. If a book is demonstrably 97% true and 3% false then makes an unfalsifiable claim we should conclude, ceteris paribus, that the claim is 97% likely to be true.
Except for the second factor, deception. Sometimes there exists a motive to deceive. Deception can lead to financial gain, political gain or other sorts of gain. Humans are teleological, or purpose-driven, entities. For this reason we rarely do things for absolutely no reason and we even more rarely do things that are against our interest. People know how trust works. Sometimes they intentionally build trust in order to deceive later on. If lying presented the biblical writers with motivation for bias then that would lower our ability to trust them. See the following where the issue of bias is dealt with by apologist J Warner Wallace:
Why can indirect reasoning be used in science but not religion? Of course it can. Here is a list of verified claims the Bible makes. It is only a fraction! Here’s another one which deals with common claims made by people to bash the Bible which are false. The Bible is highly well evidenced which makes it trustworthy. There is no reason for deception to be involved. Just because God is unfalsifiable does not make the Bible less true nor God’s existence less true! If God came like a puppy every time we blew a whistle, the way the naturalists wanted, why would he be worth calling God?