Praise be to Allah.
The rulings on those who rule by something other than that which Allaah has revealed vary according to their beliefs and actions. Whoever rules by something other than that which Allaah has revealed, thinking that it is better than the sharee’ah of Allaah, is a kaafir according to all the Muslims. The same applies to the one who refers for judgement to man-made laws instead of the laws of Allaah and thinks that this is permissible. Even if he says that referring for judgement to the sharee’ah is better, he is still a kaafir because he has permitted that which Allaah has forbidden.
But the one who rules by something other than that which Allaah has revealed because he is following his own whims and desires, or because of a bribe, or because there is enmity between him and the one against whom judgement is being passed, or for other reasons, knowing that he is disobeying Allaah by doing so and that he should refer to the laws of Allaah for judgement, is considered to be a sinner, one who is guilty of a major sin. He is considered to have committed an act of lesser kufr, lesser wrongdoing (zulm) and lesser rebellion (fisq), as was narrated from Ibn ‘Abbaas (may Allaah be pleased with him), and from Taawoos and a group of the righteous salaf. This is well known to the scholars. And Allaah is the Source of strength.