Complex sentence: Instead, they believe that those who do believe in the existence of god should be called "theists" and no specific term is required to describe their non-belief as that is the "normal" or "default" position and it is theists who are different.
Split into simpler sentences: Instead, they believe that those who "do" believe in the existence of god should be termed "theists", and that no specific term is required to describe non-belief. They consider non-belief to be the "normal" or "default" position and it is believers, or theists, who are different.