Tahkeem (judgement) is unlawful when [the judgement] is based upon a false (baatil) Sharee'ah which opposes the Book and the Sunnah, such as the laws of the Greeks (Ahkaam Yoonaan) and those of Europe, and those of the Tartars, and their various legislative codes (Qawaaneen) the source of which are their own opinions and desires. Similar to this are the various cultural and customary practices of the Bedouins. Hence, whoever made it lawful to judge (istahalla) by [any of] this in the issues pertaining to blood, or other than it is a Kaafir. Allaah the Most High said, "And whosoever does not judge by what Allaah has revealed, they are the Unbelievers" (5:44). And concerning this verse, some of the Mufassiroon have said that the kufr intended here is the kufr that is lesser than the Major kufr (kufr doona kufr al-akbar), because they understood that this verse applies to whoever judges by other than what Allaah has revealed but does not make that lawful (ghayr mustahill). But they dispute amongst themselves regarding its application in general to the mustahill (one who makes it lawful), and that the kufr in this case is the one that expels from the religion.

Source: Minhaaj ut-Ta'sees wat-Taqdees Fi ar-Radd alaa Shubuhaat Daawud bin Jarjees, (p.71).

Note: In the various statements of the scholars on this subject it is important to note that in their speech they cover and describe many different situations, including:

• Judging in an issue with other than what Allaah revealed out of hawaa (desire)
• Ruling by Secular Laws to varying degrees (in one issue, a few issues or numerous issues)
• Removing the Sharee'ah as a whole, completely, or the vast majority of it, and replacing it with secular laws