The Fi'qh on this issue is that it is unlawful to revolt against caliphs and fight them, even if they are corrupt. Fi'qh also instructs that rebellion against the caliph, even if he is unjust, when there is no mitigating pretext (or one that is patently false) or betraying said caliph because of some worldly disadvantage in remaining loyal to him is considered an enormity.

Now if you read the language carefully it prohibits rebellion against a caliph when there is NO mitigating pretext (evidence). This means that in order to justify removal of a caliph the Ummah would have to have solid evidence that he has acted against the Quran and Sunnah and that he has directed the Ummah to do contrary to the Quran and Sunnah. Rest assured, that if we had a Caliph, this would never be the case because he would be divinely ordained and would not deviate from the Quran and Sunnah. Thus it would not be necessary to challenge his rule or rebel. In this specific scenario it would definitely be haraam to rebel against his rule, but if he should be acting contrary to the interests of ALLAH, the Quran, the Sunnah and the Ummah then the language of our fiqh does not oblige us to follow him in his misdeeds. Some important ahaadeeth regarding Rulers