Why do people use the Bible to prove that God exists....and then use God to prove the bible is accurate?

Is this not the definition of circular reasoning? In science (Yup, I used the "S" word) you have to have something that can be proven independently to make your circle complete and your claim true. Why do so many Christians spout out bible verses like they are some kind of proof? They say that god is real because the scripture says so, and the scripture is accurate because God says so. Just something I noticed.