"The bible is true because it's the word of god."

 

"It's the word of god because the bible says so."

 

This is a very, very common mistake theists use in an attempt to establish facts which have no basis at all. Logic is meant to start from one or more premises, then continue one step at a time, until a logical conclusion is reached. However, this type of argument goes round in a circle, so you end up back at the start and keep going round. Basically, you are using something to prove itself, or using a lot of equivalent statements without establishing any of them are actually true. All you have done is to assume your conclusion is true before you start, and then use that to "prove" your conclusion. This is not valid. Without the assumption that at least one of statements in your circle is actually true, you have not demonstrated anything further. This is also known as "begging the question", because you are simply assuming your conclusion is true in order to demonstrate the same conclusion.

 

If you have good reason to believe one of the statements is true, then you have just shown several statements are equivalent and also true, so this is not a fallacy.

 

I think the reason this is so common is not because people intend to abuse this error, but because they have the idea that the bible is true hammered into them at a young age. It is taught as a fact, one that is beyond dispute. And so without realizing it, they assume this to be true in order to prove it is true. It's true because it says it is true. It's true because it's the word of god, and everything god says is true. But remove that assumption, that unsupported implanted statement, and the whole thing falls apart. Nothing can be evidence for itself. You would not believe I would never lie because I tell you that I never lie. To confirm the truth of anything, you need at least one (preferably more) reliable, independent sources.