Why Can’t You Use the Bible to Prove the Bible?

I would have thought that this would be obvious, but apparently it’s anything but.
So why shouldn’t you use the Bible to prove the Bible? The most obvious answer is Because if you find yourself trying to prove the Bible you are obviously talking to someone who does not accept the Bible as true. Why would some one who does not accept the Bible as true accept evidence from the Bible to prove anything? Let alone the Bible itself. If I said Thomas Paine’s The Rights of Man is true and you said “I don’t believe it is,” would you accept “Of course it is! Look at this quote here [points to quote in The Rights of Man]. It proves The Rights of Man is true,” would that be enough to convince you that The Rights of Man is true? If not, then you cannot use a text that you believe is convincing when the person you are conversing with does not think it is convincing. It won’t work. At least not until you have verified its accuracy.
The second reason why you can’t use the Bible to verify the Bible is less obvious, but is far more important if your goal is to prove the Bible. As a historian, I often find myself with a single text at the center of my research. That text usually discusses an event that I want to make some historic claim about. For example, say I want to prove that Karl Marx believed that institutionalized (organized) religion doesn’t belong in government. If it was a letter that Karl Marx wrote to Friedrich Engels that convinced me of that fact, then I would put that letter at the center of my research. Likewise, if it is the Bible that has convinced you of the truth of an event, you can put it at the center of your research. There is nothing wrong with that. However, putting a document at the center of your research does not mean using it as your only research. To go back to my Karl Marx research, I have a single document to begin with. However, that document doesn’t actually prove that Karl Marx believed that institutionalized religion does not belong in government. So I now need to prove my hypothesis with other documents. A great place to start is other work written by Karl Marx. The Capital and The Communist Manifesto may provide me with more evidence, as will other letters written by Marx. The more Marx says about institutionalized religion not belonging in government, the more likely my hypothesis is to be true. But I can’t just rely on what Marx has to say either. After all, I might be misinterpreting what he has to say. Or I may be ignoring other claims where he has said that religion should be in government. Such things are easy to miss if you are only using one type of source. So what else should I look at? Well, I should be looking at what other people had to say about Marx’s work. Did the people who knew him say that he believed institutionalized religion belonged outside of religion? Or did they say the opposite? What about other historians? What do they have to say about Marx’s view on religion in politics? Do they generally agree or disagree with me? The more evidence that I can collect in my favour, and the more flaws I can reveal in the arguments that disagree with my hypothesis, the stronger my argument is.
So why shouldn’t you use the Bible to prove the Bible? Because then you will have a very weak argument. So how do you create a strong argument? First, take the event that the Bible has convinced you of. Put the passage that convinced you at the center of your research. Singe you can’t really find other writing by the same author to corroborate that they believed what they said, you’ll have to find another, similar method of corroboration. So look for other passages in the Bible that corroborate that event. You still have a weak argument, but no it’s a bit stronger. Then you need to find others to corroborate further. Lets say you want to prove that Mary and Joseph traveled to Bethlehem for a census. Did the government who ordered the census write about it? Were there any records from the census that you could use to prove it happened? How about other travelers? Did any of them write about having to go anywhere for the same census? Did anybody from any of the neighbouring kingdoms write anything about the census? Either positive or negative? Did anybody write about Mary and Joseph specifically? For example, the wise men? Or a fellow traveler staying at the inn? Or the census takers? Anything like that would significantly strengthen your argument. But it’s still not as strong as it can be. What do other modern writers have to say about Mary and Joseph traveling for the census? Do you have a lot of support that your argument is correct? Can you verify that their arguments are a good support for your own (are they also strong arguments? Are they well researched? Have they been debunked?)? What about the people who disagree with you? Can you show why they’re wrong? If you can do all of that, then you have a strong argument.
So, to recap, don’t use the Bible to prove the Bible because the person you are trying to convince won’t be convinced, and you will have a weak argument. Use sources outside of the Bible to prove the Bible because you will convince more people and your argument will be stronger. And, as you can see, yes, this is also why you should never use any text to try to prove said text.