Question: Is one who judges by other than Allaah’s revealed Law considered a Muslim, or a disbeliever, guilty of major Kufr (major disbelief)? What about the acceptance of their deeds?
Answer: Allaah (Exalted be He) says (what means): And whosoever does not judge by what Allaah has revealed, such are the Kaafiroon (i.e. disbelievers of a lesser degree as they do not act on Allaah’s Laws). Allaah says… read more here.