Praise be to Allah.
It must be understood that one of the basic principles of faith is referring to Allaah, may He be exalted, and His Messenger (blessings and peace of Allaah be upon him) for judgement, submitting to their ruling and being content with it. Allaah says (interpretation of the meaning):
“(And) if you differ in anything amongst yourselves, refer it to Allaah and His Messenger (صلى الله عليه وسلم), if you believe in Allaah and in the Last Day. That is better and more suitable for final determination”
“But no, by your Lord, they can have no Faith, until they make you (O Muhammad صلى الله عليه وسلم) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission” [al-Nisa’ 4:65]
“Do they then seek the judgement of (the days of) Ignorance? And who is better in judgement than Allaah for a people who have firm Faith” [al-Maa’idah 5:50]
Every ruling that goes against the ruling of Allaah is a ruling of ignorance (Jaahiliyyah). Allaah says (interpretation of the meaning):
“Is not Allaah the Best of judges?” [al-Teen 95:8]
“The command (or the judgement) is for none but Allaah. He has commanded that you worship none but Him (i.e. His Monotheism); that is the (true) straight religion, but most men know not” [Yoosuf 12:40]
Thus it is clear that refusing to refer for judgement to Allaah, may He be glorified and exalted, and His Messenger (blessings and peace of Allaah be upon him), or rejecting their ruling, or believing that the ruling of someone else is better than their ruling, constitutes kufr (disbelief) and puts one beyond the pale of Islam.
Shaykh Ibn Baaz (may Allaah have mercy on him) said:
With regard to the rulings which Allaah has prescribed for His slaves and explained them in His Holy Book or on the lips of His trustworthy Messenger (blessings and peace of Allaah be upon him), such as the rulings on inheritance, five prayers, zakaah, fasting and other things which Allaah has explained to His slaves and on which the ummah is unanimously agreed, no one has the right to object to them or to change them, because this is clear legislation that was decreed for the ummah at the time of the Prophet (blessings and peace of Allaah be upon him) and after him until the Hour begins. That includes giving the male more than the female [in inheritance] among children, sons’ children, siblings through both parents and siblings through the father, because Allaah has explained it in His Book and the Muslim scholars are unanimously agreed on that, so it is obligatory to act upon that out of conviction and faith. Anyone who says that what is better is something other than that is a kaafir, and similarly the one who says that it is permissible to go against that is to be regarded as a kaafir because he is objecting to the rulings of Allaah, may He be exalted, and His Messenger (blessings and peace of Allaah be upon him) and to the consensus of the ummah. The authorities should ask him to repent if he is a Muslim; if he repents, all well and good, otherwise he is to be executed as a kaafir and an apostate from Islam, because the Prophet (blessings and peace of Allaah be upon him) said: “Whoever changes his religion, execute him.” We ask Allaah to keep us and all the Muslims safe and sound from misguiding turmoil and going against pure sharee’ah. End quote.
Majmoo’ Fataawa Ibn Baaz (4/415).