The obligation of the Islamic Aqeeda, being a basis for the Islamic State, imposes that its constitution and the rest of its rules be adopted exclusively from the Book of Allah and the Sunnah of His Messenger. Allah ordered the ruler to rule by that which Allah has revealed via His Messenger. And He (swt) has described the one who rules by other than Islam as a disbeliever if he believed in it, or believed that which Allah has revealed to His Messenger was unsuitable. He described as Asi, Fasiq and Zalim (oppressor) the one who rules by other than that which Allah has revealed and does not believe Islam to be invalid.

He (swt) said: “No by your Lord, they shall not become true believers until they make you judge in all disputes between them.” [TMQ 4:65]

And He (swt) said: “And whoever does not rule by that which Allah has revealed, they are the disbelievers (Kafirun).” [TMQ 5:44]

He (swt) said: “And whoever does not rule by that which Allah has revealed, they are the oppressors (Zalimun).” [TMQ 5:45]

And He (swt) said: “And whoever does not rule by that which Allah has revealed, they are the transgressors (Fasiqun).” [TMQ 5:47]

(From the culture of Hizb ut-Tahrir)