That is called a Circular Argument. E.g. the Bible is true because the Bible says it is true. It isn't just a bad argument, it is the worst kind of argument. It relies on Christian presuppositionalism. Every argument, question, or discussion Christians have begins with the assumption that God is real (and that their doctrine is the only true doctrine). And, it means they can never really examine the truth of their beliefs because their beliefs won't let them.
The FIRST thing you must do when you evaluate a belief is first assume everything you believe is WRONG. And then, you must ask, can I support my beliefs without any prior assumptions? Is there evidence or reason that would lead me back to those beliefs? In the case of God, and especially when considering a specific religion, when you remove the assumption of God from the equation there is no logical path that leads you back.