That’s like saying our proof for Caesar is Caesar’s writing. Now if I said Caesar’s writings were true because he said so, that would be circular reasoning. But no, they are backed up by history. In the same way, the Bible can prove Christianity, but circular reasoning is when I try and use the Bible to prove itself because it says it’s true. Although proof can lie within such as criteria of embarrassment, but mainly it’s to how it relates to what else we know about that time, which is how it lines up. Unlike, for example, the book of mormon which is completely verifiably fake as it talks about systems in an ancient america which we know didn’t exist