Is obedience to the ruler obligatory even if he does not adhere to the Book of Allah and the Sunnah of His Prophet?

It is obligatory to obey the ruler who does not rule by the Book of Allah and the Sunnah (definition) of His Messenger in matters that do not constitute disobedience to Allah and His Messenger. It is not permissible to fight him unless he reaches the point of disbelief, at which point it becomes obligatory to renounce him and his obedience is nullified.

Ruling by other than what Allah has revealed constitutes disbelief that expels one from the fold of Islam under two conditions: that the ruler is aware of the ruling of Allah and His Messenger, and that he believes that Allah's ruling is not suitable for the current time or that something else is more beneficial than it, based on the Almighty's saying: "And whoever does not judge by what Allah has revealed - then it is those who are the disbelievers" (Quran 5:44).

However, if he rules by other than what Allah has revealed, but believes that Allah's ruling is the obligatory and most beneficial, yet he violates it due to desire or injustice, then he is not a disbeliever, but rather a transgressor (fasiq) or an oppressor (dhalim). His authority remains valid, and it is obligatory to obey him in matters that do not constitute disobedience to Allah and His Messenger. It is not permissible to fight him or rebel against him unless we see clear disbelief.