Shaikh ul-Islaam Ibn Taymiyyah said:

And it is known by necessity in the Deen of the Muslims and by the agreement of all the Muslims that whoever follows a Sharee'ah other than the Sharee'ah of Muhammad then he is a Kaafir and it is like the Kufr of the one who believes in some of the Book and disbelieves in some of the Book." –"Al-Fataawa", Vol. 28/ 524

Al-Haafidh Ibn Katheer said:

"So whoever leaves the clear Sharee'ah, which was revealed to Muhammad Ibn Abdullah, the Seal of the Prophets, and takes the Hukm to other than it from the laws of Kufr which are abrogated, he has disbelieved. So what about the one who takes the Hukm to the 'Yasaaq' (the law of the Tartars which mixed Sahree'ah rulings with invented rulings) and puts it before it?! Whoever does that, he has disbelieved by the Ijmaa' of the Muslims."
– "Al-Bidaayah wa Nihaayah", Vol. 13/ 119

Shaikh 'Abdul-'Azeez Ibn 'Abdullaah Ibn Baaz said:

"There is no Eemaan for the one who believes the laws of the people and their opinions are superior to the Hukm of Allaah and His Messenger or that they are equal to it or that they resemble it or who leaves it or replaces it with fabricated laws and institutions invented by people, even if he believes that the laws of Allaah are more encompassing and more just." – "Risalaat Wujoob Tah'keem Sharee'at Allaah' Pg. 39, which follows the "Risalaat Tah'keem Al-Qawaneen" Published by "Daar Al-Muslim"

Shaikh Muhammad Ibn Saalih al-'Uthaymeen said:

"The first type is when the Hukm of Allaah is removed and replaced with another Taghuutee Hukm, so that the Hukm of the Sharee'ah is eliminated between the people and he puts in its place another Hukm from the fabrication of the humans and they remove the laws of the Sharee'ah concerning the Mu'amilah (i. e. the general actions between people) and they put in its place fabricated laws and this, without doubt, is Istib'daal (i. e. replacement) of the Sharee'ah of Allaah subhaanahuu wa-ta'ala, with other than it. And this is Kufr which removes one from the Milla because this person put himself at the level of the Creator because he shara'a (legislated) for the slaves of Allaah that which Allaah ta'ala did not give permission for and that is Shirk in His, ta'ala's saying: "Or have they partners with Allâh (false gods), who have instituted for them a religion, which Allâh has not allowed?" (Ash-Shu'ara, 21) – "Fiqh Al-' Eebaadaat", #60

Shaikh Muhammad Naasiruddin Al-Albaanee:

Who said, in one of his earlier cassette recorded lessons, wherein he is describing an argument he had with someone about the Takfeer of Mustafah Ataturk, the secularist who converted the constitution of Turkey from the Hanafee code Sharee'ah, to the man-made laws. So Shaykh Al-Albaanee said:

"I made clear to him (i. e. his opponent) that the Muslims did not make Takfeer to Ataturk who was Muslim. No. (They did so) when he freed himself from Islaam when he implicated upon the Muslims an institution other than the institution of Islaam. And from that was the example of his equalizing between the inheritance of the male and the female. But Allaah says according to us, 'And for the male is the share of two females. ' And then he obligated upon the Turkish masses, the Qobah (i. e. a Turkish-style hat)."
– "Fataawa Ash-Shaykh al-Albaanee wa-Maqara'netihah bi'Fatawaa Al-' Ulaama", Pg. 263 from his cassette #171.

Shaikh Mahmood Shaakir said:

So their question wasn't the 'Eebadeeyah's question to Abee Majliz about the Tafseer of this Ayaah – about that which the Mub'tadah of our time agree with concerning the judgement in money andblood with a law that opposes the Sharee'ah of the people of Islaam and not concerning implicating a law upon the people of Islaam and forcing them to take the judgement to other than the rule of Allaah in His Book and upon the tongue of His Prophet. So this action is turning away from the Hukm of Allaah and from His Deen and putting the laws of the Kuffar above the law of Allaah, subhaanahuu wa-ta'ala and this is Kufr. No one from the people of the Qiblah with their difference, doubts the Kufr of the one who says or calls to this." – From his commentary of At-Tabaree (" Tafseer At-Tabaree" Vol. 10/ 348)

Alaamah Muhammad Al-Ameen Ash-Shanqeetee said:

"And with these Heavenly texts that we have mentioned, it becomes quite clear that the ones who follow the fabricated laws, which the Shaytaan has legislated upon the tongues of his 'Auliya and which oppose that which Allaah, jala-wa'ala has legislated upon the tongues of His Messengers, peace be upon them, that no one doubts their Kufr and their Shirk except him who Allaah has removed his sight and has blinded them to the light of the revelation as they are!"
– "Adhwaa Al-Bayaan", Vol. 4/ 82-85

Shaikh Saalih al-Fowzaan said:

"So whoever takes the Hukm to other than the legislation of Allaah from all of the institutions and the man-made laws, then has taken the implicators of these laws and the ones who rule with them as partners with Allaah in his legislation. He, ta'ala said: 'Or do they have partners who have legislated for them what Allaah has not allowed? ' And He said: 'And if you obeyed them, then you are Mushrikeen. '
– "Al-Irshaad ila'Saheeh Al-' Atiqaad ", Vol. 1/ 72

And then again, after narrating what Al-Haafidh Ibn Katheer wrote concerning the Tartar's and "Al-Yasiq", he said, "And the likes of the law that he mentioned from the Tartars, and judged upon with Kufr, those who put in the place of the Islaamic Sharee'ah, are the fabricated laws, which have – in our time – been established as sources of laws in many countries and the Islaamic Sharee'ah has been disregarded in favor of them except in what they call 'personal matters'."
– "Al-Irshaad ila'Saheeh Al-' Atiqaad ", Vol. 1/ 74

Shaikh 'Umar al-Ashqar said:

And from this explanation it becomes clear to us that there are two types of people who have fallen into Kufr about which there is no doubt. The first, the ones who legislate that which Allaah did not reveal, and those are the ones who fabricate the laws that oppose the legislation of Allaah they implicate it upon the people and the Ijmaa' is upon their Kufr without doubt." – "Al-Sharee'ah Al-Eelaheeyah", Pg. 179

'Alaamah Muhammad Ibn Ibraaheem Aal-Ash-Shaykh said:

"… The fifth, and it is the greatest and the most encompassing and the clearest opposition of the Sharee'ah and stubbornness in the face of its laws and insulting to Allaah and His Messenger and opposing the courts of the Sharee'ah on their roots and branches and their types and their appearances and judgements and implementations the references and their applications. So just like the courts of the Sharee'ah there are references, all of them returning back to the Book of Allaah and the Sunnah of His Messenger like that, these courts have references, which are laws that are assembled from many legislations and laws like the laws of France and America and England and other laws and from the Metha'haab of some of the innovators who claim to be under the Sharee'ah.

And these courts are now fully operational in the settlements of Islaam, people entering them one after another, their rulers judge upon them with what opposes the Sunnah and the Book with the rules of that law and they impose that on them and approve it for them. So what Kufr is there beyond this Kufr and what nullification of the Shahaadah of Muhammadar Rasool-Allaah is there beyond this nullification?!
– "Tah'keem Al-Qawaneen"

Shaikh 'Abdur-Razzaaq al-'Afeefee said:

"Thirdly: The one who is attributed to Islaam and knows its laws and then fabricates for the people, laws and makes them an institution for them to conduct themselves by and to take their judgements to and he knows that it opposes the laws of Islaam. Then he is a Kaafir out of the Milla of Islaam. And like that is the Hukm concerning the one who orders a committee or committees to be formed for that and the one orders the people to take their judgements to these institutions or laws or makes them take the judgements to them, while he knows that they oppose the Sharee'ah of Islaam. And like that is the one who judges with it and implicates it upon the matters and the one who obeys them in these judgements out of his own choice, while he knows that it opposes Islaam. So all of these are partners in their turning away from the Hukm of Allaah."
– "Shubu'haat Howl As-Sunnah Wa-Risalaat Al-Hukm bi'Ghayr ma'Anzaal-Allaah", Pg. 64

Imaam Ahmad Shaakir said:

"The matter in these fabricated laws is clear with the clearness of the sun. It is clear Kufr and there is nothing hidden about it and there is no excuse for anyone who attributes themselves to Islaam, whoever they may be, to act according to it or to submit to it or to approve of it. So each person should beware and every person is responsible for himself. So the 'Ulaama should make the truth clear and tell what they have been ordered to tell without concealing anything."
– "Umdaat At-Tafseer Mukhtaasir Tafseer Ibn Katheer of Ahmad Shaakir", Vol. 4/ 173-174

Imaam Ibn Jareer At-Tabaree said:

"He ta'ala says, whoever conceals the Hukm of Allaah, which He revealed in His Book and made it a law between the slaves – so he hides it and rules with other than it like the Hukm of the Jews concerning the married fornicators with whipping of the guilty and blackening their faces and concealing the Hukm of stoning and like their judging upon some of their murdered with full blood-money and some with half of their blood-money. And concerning the noble people, they would have Qisaas but the commoner would only get the blood money. But Allaah made all of them equal in the Tauraat: …such are the Kâfirûn. They are the ones who concealed the truth, which was upon them to uncover and make clear. And they hid it from the people and they showed something different to the people and they judged according to that (changed Hukm) because of a bribe they took from them." (* So the point of At-Tabaree here is that he considers this Ayaah general for anyone who does what the Jews did and hold this Ayaah meaning of Kufr Akbaar upon anyone who does what they did.)
– "Tafseer Al-Tabaree" Vol. 4/ 592

Shaikh Muhammad al-Ghunaymaan was asked:

"The one who leaves the Hukm by what Allaah revealed; if he makes the general judgements with the fabricated laws, does he disbelieve? And is there a difference between that and the one who judges with the Sharee'ah but then he opposes the Sharee'ah in some of the matters due to desire or bribery or other than that?"

So he answered, "Yes, it is Waajib to differentiate between them. There is a difference between the one who throws away the Hukm of Allaah, jala-wa'ala and replaces it with the judgements with the laws and the judgement of mankind. This is Kufr, which takes one outside the Milla of Islaam. But the one who is Multazim (i. e. religiously committed) upon the Deen of Islaam except that he is disobedient and a Thaalim by following his desires in some of the Ah'kaam and goes after a benefit from the Dunyah, while accepting that he is Thaalim with this, then this is not Kufr, which takes you out of the Milla. And whoever sees the Hukm with the laws to be equal to the Hukm of the Shara' and makes it Halaal, then he also disbelieves with the Kufr that takes one outside the Milla, even if it is in one instance."
– "Mujaalit Al-Mishkaat", Vol. 4/ 247

Shaikh Ibn Qaasim said:

Like the ones who rule with the laws of Jahiliyyah and the international laws, rather, even one who rules by other than what Allaah revealed, whether he rules with the laws or with something which has been invented that is not from the Shara' or affirmed in the Hukm, then he is a Taghuut from the greatest Tawagheet."
– From his commentary on "Usool ath-Thalaathah", Pg. 96

Shaikh Hammad Ibn 'Ateeq an-Najdee said:

"And the Fourteenth Matter is Taking the Hukm to Other than the Book of Allaah and His Messenger. " And then he mentions the Fatwaa of Ibn Katheer under the Ayaah: "Is it the Hukm of Jahileeyah which they seek?", which we have narrated earlier. Then he said, "And like this is what the general people of the Bedouins and those like them fell into with regards to taking the Hukm to the customs of their forefathers and that which their ancestors established from the accused customs, which they label 'The Sharee'ah of Reefawah' they put it before the Book of Allaah and the Sunnah of His Messenger. So whoever does that; then he is a Kaafir and it is Waajib to fight him until he returns to the Hukm of Allaah and His Messenger."
– "Majmoo'at At-Tawheed ", Pg. 412

Shaikh 'Abdullaah Ibn Humayd said:

And whoever makes a general legislation (Tashree' Al-' Aam) and implicates it upon the people which opposes the Hukm of Allaah, then this one leaves the Milla as a Kaafir."
– "A'hameeyaat Al-Jihaad" by 'Alee bin Nafee' Al-' Ilyaanee Pg. 196

Shaikh Muhammad Hamad al-Faqeeh said:

And like or (even) worse than this are the ones who take the words of the Kuffaar as laws, which they judge with in matters concerning blood and wealth and they put that before that which they know and that is has been made clear to them from the Book of Allaah and the Sunnah of His Messenger. So he, without a doubt, is a Mortad if he continues upon that and does not return to the Hukm of what Allaah revealed and he will not be benefited by any name which he labels himself with and neither by any outward action that he does from Salaat or Siyaam or anything else!"
– From the Hamish (i. e. margins) of "Fat'h Al-Majeed", Pg. 406

'Abdul-Lateef Ibn 'Abdur-Rahmaan (Muhammad Ibn 'Abdul-Wahhab's great-grandson):

When asked concerning what the Bedouins judge with according to the customs of their fathers and grandfathers. "Do we label them with Kufr after it is made clear to them (that this is not permissible and when they continue)?"

So he answered, "Whoever takes the judgement to other than the Book of Allaah and the Sunnah of His Messenger after it is made clear to him (that this is not permissible), then he is a Kaafir. He, ta'ala said: 'And whosoever does not judge by what Allâh has revealed, such are the Kâfirûn. ' (And He ta'ala also said:) 'Is it other than the Deen of Allaah that they seek? '(And He ta'ala also said:) 'Have you seen those (hyprocrites) 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 Tâghût (false judges, etc.) while they have been ordered to reject them. ' 'And the Ayaat with this meaning are many."
– "Dur'ur As-Saneeyah fi'Al-Ajweebah An-Najdeeyah", Vol. 8/ 231 Published by "Dar Al-Iftaa' bil'Saudeeyah" 1385 H

Shaikh Muhammad Shaakir Ash-Shareef said:

In his "Chapter concerning making clear when the one who rules by other than what Allaah revealed is a Kaafir; with the Kufr that does not take one outside the Milla."

Then he said, "He does not disbelieve with three conditions:
a) That he is Multazim (i. e. religiously committed) and accepts upon the outside and the inside every Hukm or Tashree' (legislation) which has come from Allaah subhaanahuu wa-ta'ala or His Messenger -sallallaahu 'alayhi wa sallam.
b) That he accepts and confesses that he has left the Hukm with what Allaah subhaanahuu wa-ta'ala has revealed in that matter or that specific instance that he judges in that he is sinful and that his Hukm is a mistake and that the Hukm of Allaah is the correct.
c) That the opposing Hukm is a Hukm in specific instances and not in full general matters and this third condition is the one, which many of the contemporary people have not understood and paid attention to.
– "In'Allaah Huu'al-Haakim", Pg. 88-91 published by "Daar Al-Waton", 1413 H.

Shaikh Saalih Ibraaheem al-Layhee:

Who said, concerning the meaning of the "Daleel", "So the ruling with the fabricated laws, which oppose the Islaamic Sharee'ah is atheistic and Kufr and Fasad and Thulm among the slaves because the security is not ensured and the Shara'ee rights are not preserved except by acting upon the Islaamic Sharee'ah in its entirety in the 'Aqeedah and worship and ruling and etiquettes and the conduct and institutions, because the 'Ruling by Other Than What Allaah Revealed' is ruling with a created action upon a creation like it. And it is ruling with the laws of the Taghoot and there is no difference between the individual conditions and the general and specific; and whoever differentiates between them in the Hukm, then he is an atheist/ Zandeeq (i. e. Hypocrite in denial)/ Kaafir in Allaah Al-' Atheem!"
– "As-Salsabeel ", Vol. 2/ 384, which is his commentary upon "Zaad Al-Mutaqnah ".