Imam Ibn Abil Izz al-Hanafee stated, “Here there is a matter which has to be understood and that is: ruling by other than what Allah has revealed can be kufr which expels one from the religion (major kufr); it can be disobedience, major or minor.
So this all depends on the condition of the ruler: if he believes that ruling by other than what Allah has revealed is not an obligation, or that he has a choice in a matter, or that he mocks it while admitting that it is the rule of Allah, then this is major kufr (which expells a person from Islam);
if he believes that it is an obligation to rule by what Allah has revealed and this is his knowledge of the situation, yet he does not rule by it, along with his admittance that he deserves punishment, then this is disobedience and such a person is a disbeliever for committing kufr in the figurative sense or has committed minor kufr (which does not expell a person from Islam);
if he is ignorant of the rule of Allah, while he exerts great efforts in trying to know the ruling yet makes a mistake, then this is one who has been mistaken. He has a reward for his ijtihad and his error is forgiven.”
Ref: Sharh ul-Aqeedah at-Tahawiyyah by Imam Ibn Abil Izz al-Hanafee, pp.323-324
Posted by Abdul Kareem Ibn Ozzie