The judge by what Allah has not revealed:
Belief in Allah and worshipping Him necessitate submission to His Judge, satisfaction with His Shari’a (Legislation), and resorting to His Book (Qur’an) and His Messenger’s Sunnah (Tradition) in disagreements. These disagreements may be in sayings, fundamentals, Litigation, blood, properties, and other legitimate rights. This is because Allah is the Judge, and the Rule must be His. So human rulers have to judge by what Allah has revealed and subjects have to resort back to this judgement, which is stated in the Book of Allah and in the Tradition of His Messenger. Allah said concerning the rulers, -meaning- {Allah does command you to render back your Trusts to those to whom they are due; and when you judge between man and man, that you judge with justice}. And said concerning the subjects, -meaning- {O you who believed! Obey Allah, and obey the Messenger, and those charged with authority among you. If you differ in anything among yourselves, refer it to Allah and His Messenger (Qur’anic and Sunnah Scriptures), if you do believe in Allah and the Last Day: that is best, and most suitable for final determination}. After all, He clarified that there exists no faith when there is no resorting to what Allah has revealed, and said, -meaning- {Has you (O Muhammad (saw) not turned your vision to those who declare that they believe in the revelations that have come to you and to those before you? Their (real) wish is to resort together for judgment (in their disputes) to the Evil One, though they were ordered to reject him. But Satan’s wish is to lead them astray far away (from the Right)}. Until He said, -meaning- {But no, by your Lord, they can have no (real) Faith, until they make you judge in all disputes between them, and find in their souls no resistance against your decisions, but accept them with the fullest conviction}.
Therefore, Allah has denied certainly supporting His denying with an Oath the faith of this who refused to resort to the Prophet’s judge and be satisfied with it. So is the case with those rulers who do not rule by what Allah has revealed and He confirmed their injustice and going out of the obedience, He said, -meaning- {And if any do fail to judge by (the light of) what Allah has revealed, they are (no better than) Unbelievers}.
-meaning- “And if any fail to judge by (the light of) what Allah has revealed, they are (no better than ) wrongdoers.”
-meaning- “And if any do fail to judge by (the light of) what Allah has revealed, they are (no better than ) those who rebel.”
Resorting to what Allah has revealed in His Legislation is by all means necessary in case of disagreements between scholars of Islam. So that nothing should be acceptable except what is proved by evidence from the Book (Qur’an) or the Sunnah (Prophet’s Tradition) without being fanatic to or sided. Furthermore, this should be applicable in case of legitimate procedures and litigation in addition to the rest of the human rights. In fact, it should not be limited to the personal law (marriage and divorce etc.) just like some countries whose rules pretend to be Muslims; actually, Islam is part and parcel, Allah (swt) said, -meaning- {O you who believe! Enter into Islam whole-heartedly}, and said, -meaning- {Then is it only a part of the Book that you believe in, and do you reject the rest?}.
Similarly, those who follow some scholars’ diligence have to attribute these scholars’ opinions to the Qur’an and the Sunnah and adopt them according to their agreement with these two principles. Especially when it is concerned with doctrines because these scholars recommended that. However, the one who contradicts this principle is by all means not following their diligence, and he is one of those whom Allah mentioned, -meaning- {They take their priests and their anchorites to be their lords in derogation of Allah, and (they take as their Lord) Isa (Jesus), the son of Mary}.
This Verse is not specially descended for Christians, but rather concerned with whomever followed this path and broke the orders of Allah and His Messengers. Therefore, whosoever judged between people by what Allah has not decreed or suited oneself in judgement has excluded himself from faith and Islam even if he claimed being faithful. This is because Allah denied the belief of those who did that by saying, -meaning- “claim”, and ensured that by saying, -meaning- {though they were ordered to reject him}.
Verily, because the disbelief in Taghut (anything wrongly worshipped or obeyed other than the Real God, Allah) is a basis of Tawhid (Monotheism), as stated in Surat Al-Baqarah (The Cow). Therefore, if this basis is not achieved there exists no Tawhid at all, and when Tawhid exists not, then deeds are nullified for Allah (swt) said, -meaning- {Whoever rejects Taghut (those who are obeyed or worshipped besides Allah) and believes in Allah has certainly grasped the most trustworthy hand-hold}, because the resorting to the Taghut as a judge is a belief in it.
Verily, the resorting to what Allah has revealed in His Book and to His Messenger as a Sunnah is a doctrine according to which belief is approved or denied; however, it should not be applied for the fact that it is more applicable for the human benefits and whatsoever. This should be quite understood as the basic doctrine according to which Muslim would be rewarded or punished, In fact, Allah dispraised those who resorted to the Judge of Allah and His Messenger just because its benefits and said, -meaning- {When they are summoned to Allah and His Messenger (Muhammad (saw), in order that He may judge between them, behold, some of them decline (to come). But if the right is on their side, they come to him with all submission}.
So they do not really care except for what it suits them, and everything else is neglected because they did not consider the resorting to the Messenger’s judge as worship.