Allaah, the Exalted, said: "And He who does not rule by what Allaah has revealed it is they who are the unbelievers." (5:44)
This Ayah signifies that ruling by laws other than those revealed by Allah constitutes infidelity. This type of infidelity is either major which entails apostasy, or minor which does not constitute apostasy depending on the ruler's attitude. If he believes that the laws of Allaah are not binding, and that he has a choice, or he took them lightly, or believed that man-made laws are better than the Divine laws, because the former are not suitable for this time and age, or if he wants to satisfy the infidels and hypocrites by so doing, then he commits major infidelity. But if he believes that the laws of Allaah are binding and knows the respective laws are relevant to the case he is handling, and yet he opted to rule by otherwise, recognizing that his deed is wrong, and would be subject to punishment, then such a ruler is disobedient who has committed minor Kufr or disbelief. But if he was ignorant of the respective Divine law and tried his best and utmost seeking it, but missed it, he would have committed an error, but he would be rewarded for exerting the effort to reach the proper judgment and, whose error would be forgiven [At-Tahawiyah p.363-364].
This applies to cases that concern individuals, but ruling on cases that concern the public is a different case. Shaikhul Islam b. Taymiyyah said: "If the ruler was religious and yet ruled ignorantly without knowledge, he would end up in Hell-Fire. But if he is aware of the Divine laws and yet he ruled by other than the truth of which he is, aware, he would belong to the people of Hell-Fire. And if he rules unjustly and without knowledge, he would be more deserving of Fire. This in case he judges in a case of an individual, but if he judges in a case which concerns the public, and altered the truth into falsehood, and the falsehood into truth, the Sunnah into Bid'ah, or the Bid'ah into Sunnah, and the good into evil and the evil into good, and opposes the commands of Allaah and His Messenger and commands what Allaah and his Messenger forbid, then this is another type to be judged by Allaah, the Rubb of the Worlds, and the God of the Messengers, the Proprietor of the Day of Requital, to Whom belongs all the praise in the beginning and the end. [Majmoo' al-Fatawa, p.388]
Allaah says: "The judgement is His, and to Him you shall return." (28:88) Allaah says: "He it is Who sent His Messenger with guidance and the deen of the truth, to make it prevail over all other religions. And Allaah is sufficient as a Witness." (48:28)
Shaikhul Islam Ibn Taymiyyah went on to say: "There is no doubt that he who does not believe in the incumbency of ruling by what Allaah sent to His Messenger is an infidel. So he who deems it lawful to judge between people according to what he deems as justice without adhering to what Allaah sent down is an infidel too.
There is not a nation but enjoins ruling with justice, although justice according to their religion is what their leaders or chiefs consider as such. There are many who claim to be Muslims, rule by their mores and customs which is not revealed by Allaah, such as the customs of the Bedouins, which they inherited from their forefathers who were obeyed rulers. They believe that such customs must be applied, excluding the Book and the Sunnah, and such belief constitutes infidelity.
There are many people who accept Islam, but do not rule except by the prevailing customs that are commanded by obeyed lords. Such people know that it is not permissible to rule by other than what Allaah revealed, but do not apply them, rather they deem as lawful to rule by other than what Allaah sent down, they are infidels." [Minhaaj as-Sunnah]
Shaikh Muhammad b. Ibraheem (the late Mufti of Saudi Arabia who is succeeded by Shaikh Bin Baz) asserted: "As for that which is described as lesser infidelity, it is when he refers the dispute to other than the Book of Allaah knowing that he is disobeying Allaah by doing so, and that the ruling of Allaah is the truth, and He does it once. Such a person would not be committing major infidelity. As for those who legislate laws and make others obey them, this constitutes infidelity, even if they claim that they made a mistake, and that the laws of Allaah are more just; such is considered as an infidelity which entails apostasy." [Fatawa ash-Sheik Muhammad b. Ibreheem aal-Ash-Sheik]
By asserting this Sheik Muhammad b. Ibraheem, may Allaah grant him His Mercy, made distinction between the partial ruling of no recurrence, and between the general ruling which is used as reference to all judgments or most of them, and he decided that such infidelity constitutes absolute apostasy. That is because he who discards the Islamic Shar'iah and replaces it with man-made laws, this is a proof that he considers the man-made laws as more appropriate and better than the Shar'iah, which is beyond doubt a major infidelity which constitutes apostasy and contradicts Tawheed (the belief in the Oneness of Allaah).