No, it's not. First of all, the definition of the Big Bang includes that it didn't start itself. Secondly, believing in God doesn't mean you can't believe in old earth or the Big Bang. I think that was how God created the universe.
It does mean you can't believe the Bible, though.
You need to do slightly more than just say 'No, it's not'. And there're several entirely naturalistic hypotheses promoted by theoretical physicists and Cosmologists that try to explain how our Universe came into existence.
Being at the very early stage where we're struggling to understand how the Universe could have emerged from a natural cause, doesn't mean that no such explanation can be found.