Allaah, the Exalted, said: “And He who does not rule by what Allaah has revealed it is they who are the unbelievers.” (5:44)
This Ayah signifies that ruling by laws other than those revealed by Allah constitutes infidelity. This type of infidelity is either major which entails apostasy, or minor which does not constitute apostasy depending on the ruler’s attitude. If he believes that the laws of Allaah are not binding, and that he has a choice, or he took them lightly, or believed that man-made laws are better than the Divine laws, because the former are not suitable for this time and age, or if he wants to satisfy the infidels and hypocrites by so doing, then he commits major infidelity. But if he believes that the laws of Allaah are binding and knows the respective laws are relevant to the case he is handling, and yet he opted to rule by otherwise, recognizing that his deed is wrong, and would be subject to punishment, then such a ruler is disobedient who has committed minor Kufr or disbelief. But if he was ignorant of the respective Divine law and tried his best and utmost seeking it, but missed it, he would have committed an error, but he would be rewarded for exerting the effort to reach the proper judgment and, whose error would be forgiven [At-Tahawiyah p.363-364].