Really, in order to believe in something, you need some kind of proof. For example, I just believe and this faith gives me and what kind of confidence and belief in a better life in the future. Therefore, what proof of the existence of God is needed, if it is not material.
Actually, you don't need any kind of proof to believe in something, you simply need a source of information you trust.
If we take the Bible as a source of information, then this book can not be authoritative for everyone, because people also wrote it. But this is only relevant for those who seek or doubt. And for those who believe, this book is of great value.