A positivist is someone who believes in Auguste Comte's philosophy of Positivism. Positivism is the belief that all things that can be observed are real, thereby Comte was "positive of reality". A skeptic, in the way you are probably describing, is someone who has doubts of religious claims. Someone who doubts non-believers would therefore most likely be a religious believer. Positivism spawned the Religion of Humanity, a mostly failed project by Comte that was an attempt of a secular religion. A skeptic's doubter is most likely to be spiritual, rather than secular. A positivist is not a skeptic's doubter.