Ruling by that which Allaah has revealed is part of Tauhid al-Ruboobiyyah (Oneness of the divine Lordship), because it is implementing the ruling of Allaah as is implied by acceptance of His Lordship (Ruboobiyyah) and His perfect sovereignty and control. Hence Allaah calls those who are followed in a manner contrary to what He has revealed arbaab (lords) of their followers. Allaah says (interpretation of the meaning):
“They (Jews and Christians) took their rabbis and their monks to be their lords [arbaab] besides Allaah (by obeying them in things which they made lawful or unlawful according to their own desires without being ordered by Allaah), and (they also took as their Lord) Messiah, son of Maryam (Mary), while they (Jews and Christians) were commanded [in the Tawraat (Torah) and the Injeel (Gospel)] to worship none but One Ilaah (God — Allaah) Laa ilaaha illa Huwa (none has the right to be worshipped but He). Praise and glory be to Him (far above is He) from having the partners they associate (with Him)”
Allaah calls those who are followed “lords” because they are appointed as lawgivers along with Allaah, and He calls the followers “slaves” because they submit to them and obey them in going against the ruling of Allaah, may He be glorified and exalted.
‘Adiyy ibn Haatim said to the Messenger of Allaah (peace and blessings of Allaah be upon him): “But they do not worship them.” The Prophet (peace and blessings of Allaah be upon him) said: “Nay, they forbid to them that which is permissible and they permit to them that which is forbidden, and they follow them; that is how they worship them.”
Once you understand this, you should realize that with regard to the one who does not rule by that which Allaah has revealed and wants rulings to be referred to someone other than Allaah and His Messenger, there are verses which state that he has no faith, and verses which state that he is a kaafir, zaalim (wrongdoer) and faasiq (rebellious evildoer).
As for the first category (i.e. the verses which state that he has no faith):
For example, Allaah says (interpretation of the meaning):
“Have you not seen those (hypocrites) who claim that they believe in that which has been sent down to you, and that which was sent down before you, and they wish to go for judgement (in their disputes) to the Taaghoot (false judges) while they have been ordered to reject them. But Shaytaan (Satan) wishes to lead them far astray.
61. And when it is said to them: ‘Come to what Allaah has sent down and to the Messenger (Muhammad صلى الله عليه وسلم),’ you (Muhammad صلى الله عليه وسلم) see the hypocrites turn away from you (Muhammad صلى الله عليه وسلم) with aversion.
62. How then, when a catastrophe befalls them because of what their hands have sent forth, they come to you swearing by Allaah, ‘We meant no more than goodwill and conciliation!’
63. They (hypocrites) are those of whom Allaah knows what is in their hearts; so turn aside from them (do not punish them) but admonish them, and speak to them an effective word (i.e. to believe in Allaah, worship Him, obey Him, and be afraid of Him) to reach their innerselves.
64. We sent no Messenger, but to be obeyed by Allaah’s Leave. If they (hypocrites), when they had been unjust to themselves, had come to you (Muhammad صلى الله عليه وسلم) and begged Allaah’s forgiveness, and the Messenger had begged forgiveness for them, indeed, they would have found Allaah All-Forgiving (One Who forgives and accepts repentance), Most Merciful.
65. 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”
Allaah ascribes to these people who claim to have faith when they are hypocrites several characteristics:
1.They want to refer for judgement to al-taaghoot, which is everything that goes against the ruling of Allaah and His Messenger (peace and blessings of Allaah be upon him), because going against the ruling of Allaah and His Messenger is tughyan and transgression against the ruling of the One to Whom belongs judgement and all things return to Him, namely Allaah. Allaah says (interpretation of the meaning): “Surely, His is the creation and commandment. Blessed is Allaah, the Lord of the ‘Aalameen (mankind, jinn and all that exists)!” [al-A’raaf 7:54].
2.If they are called to that which Allaah revealed to the Messenger, they refuse and turn away.
3.If a calamity befalls them because of what their hands sent forth, such as being exposed for their wrongdoing, they come swearing that they only meant good will and conciliation, as in the case of those who reject the rule of Islam today and govern according to laws that go against it, claiming that this is good and suited to modern circumstances.
Then Allaah warns these people who claim to be believers but who bear these characteristics that He knows what is in their hearts and what they are concealing of things that are different from what they say. And He commands His Prophet to admonish them and speak to them an effective word. Then He explains the wisdom behind sending the Messenger to be the only one who to be obeyed and followed, and no one else, no matter how strong their ideas and how vast their understanding. Then Allaah swears by His Lordship to His Messenger, which is the most special kind of Lordship and which points to the truth of His message, confirming that faith cannot be valid except with three things:
1.Referral for judgement in all disputes must be made to the Messenger of Allaah (peace and blessings of Allaah be upon him).
2.They should open their hearts to his judgement and there should be no objection or resistance to it in their hearts.
3.There must be full submission and acceptance of his ruling and it must be implemented with no hesitation or deviance.
As for the second category (verse which say that he is a kaafir, zaalim and faasiq):
For example, Allaah says (interpretation of the meaning):
“And whosoever does not judge by what Allaah has revealed, such are the Kaafiroon (i.e. disbelievers — of a lesser degree as they do not act on Allaah’s Laws)”
“And whosoever does not judge by that which Allaah has revealed, such are the Zaalimoon (polytheists and wrongdoers — of a lesser degree)”
“And whosoever does not judge by what Allaah has revealed (then) such (people) are the Faasiqoon [the rebellious i.e. disobedient (of a lesser degree)] to Allaah”
Can these three characteristics be applied to one person, i.e., is the one who does not rule by what Allaah has revealed a kaafir, zaalim and faasiq? Because Allaah has described the kaafirs as being wrongdoers and evildoers, as He says (interpretation of the meaning): “And it is the disbelievers who are the Zaalimoon (wrongdoers)” [al-Baqarah 2:254] and “Certainly they disbelieved in Allaah and His Messenger, and died while they were Faasiqoon (rebellious, — disobedient to Allaah and His Messenger صلى الله عليه وسلم)” [al-Tawbah 9:84], so every kaafir is a wrongdoer and evildoer – or do these characteristics apply to those described only insofar as they do not rule by that which Allaah has revealed? The latter is more likely in my view, and Allaah knows best.
We say: The one who does not rule by that which Allaah has revealed because he regards it as insignificant, or he looks down on it, or he believes that something else is better and more beneficial to people, is a kaafir in the sense of being beyond the pale of Islam. Among these are those who promulgate for the people legislation that goes against Islamic laws to be a way by which people live. They only promulgated that legislation which goes against Islamic laws because they believe that it is better for people, because it is well known on the basis of common sense [?] that people do not turn away from one way to another way that is different unless they believe that the one they turned to is better and that the one they turned away from is lacking.
If a person does not rule by that which Allaah has revealed, but he does not regard it as insignificant, look down on it or believe that something else is better than it and more beneficial for people, rather he rules by something else by way of wronging those who are under his rule and out of vengeance and so on, then he is a wrongdoer but he is not a kaafir, and the level of his wrongdoing varies according to the alternative by which he rules and the means of ruling.
If a person does not rule by that which Allaah has revealed, but he does not regard it as insignificant, look down on it or believe that something else is better than it and more beneficial for people, rather he rules by something else so as to fulfill the wish of those who are ruled, or because of a bribe or for some other worldly reason, then he is a faasiq (rebellious evildoer) but he is not a kaafir, and the level of his wrongdoing varies according to the alternative by which he rules and the means of ruling.
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said concerning those who take their rabbis and monks as lords instead of Allaah that they are of two types:
1.Those who know that they changed the religion of Allaah and they follow them in those changes, and believe in the permissibility of what was forbidden and the prohibition of what Allaah has permitted, following their leaders, even though they know that they have gone against the religion of the Messengers. This is kufr and Allaah and His Messenger regard it as shirk.
2.Those who believe in the permissibility of what was forbidden and the prohibition of what Allaah has permitted, but they obeyed them in disobeying Allaah, as the Muslim does when he commits sins that he believes are sins. They come under the same ruling as other sinners. End quote.
Shaykh Muhammad ibn Uthaymeen (may Allah have mercy on him). Fataawa al-‘Aqeedah (p. 208-212).