​Some people say something like the following:

"You can't quote the Bible to prove the Bible because it's circular reasoning."

There's something about what they're saying that's right. The following is a bad argument:

1. The Bible says it's the word of God.
2. I can trust what it says, since it's the word of God.
3. Therefore, I can trust it when it says it's the word of God, so I should believe that it's the word of God.

However, that's not the only thing someone can mean when saying that the Bible can count as evidence for Christianity. I have in mind a very different kind of argument. What Christians call the Old Testament (and what scholars today call the Hebrew Bible) could have taken something like 1500 years to produce, perhaps shorter but certainly well over 1000 years even by liberal estimates (though how much of it one says is early depends on one's presuppositions). Adding in the New Testament (or Greek Bible, if you prefer that name) brings it to 1500-2000 years. Think about what's happened in the last 2000 years.
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Two related arguments come to mind. One has to do with prophecy. The other is from the unity of the Bible.