You are either a theist or not. If you believe in a god you are a theist. If you don't believe in a god you aren't. When one does not believe in a god one is an atheist.
The question whether any of the religions' gods exist has (for all the ones I know of) been answered for sometime. For example the many gods that the various "Abrahamic" religions claim exist we know don't exist as the empirical evidence they claim for such existence is on examination lacking.
"Could a god exist?" is a very different question and it requires a definition for the word god before it has meaning. Even if your answer is "I don't know" you are an atheist as you do not have a belief in a god.