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

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Al-Haafidh Ibn Katheer said:

"So whoever leaves the clear Shari'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 'Yasiq' (the law of the Tartars which mixed Shari'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

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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 Allah 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 Allah are more encompassing and more just."
– "Risaalat Wujoob Tah'keem Sharee'at Allah' Pg. 39, which follows the "Risaalat Tah'keem Al-Qawaneen" Published by "Daar Al-Muslim"

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Shaikh Muhammad Ibn Saalih al-'Uthaymeen said:

"The first type is when the Hukm of Allah is removed and replaced with another Taghuutee Hukm, so that the Hukm of the Shari'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 Shari'ah concerning the Mu'amalah (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 Shari'ah of Allah 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 Allah that which Allah 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-'Ebaadaat", #60

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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 Allah, 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 Allah has removed his sight and has blinded them to the light of the revelation as they are!"
– "Adhwaa Al-Bayaan", Vol. 4/ 82-85

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Shaikh Saalih al-Fowzaan said:

"So whoever takes the Hukm to other than the legislation of Allah 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 Allah in his legislation. He, ta'ala said: 
'Or do they have partners who have legislated for them what Allah 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 Shari'ah, are the fabricated laws, which have – in our time – been established as sources of laws in many countries and the Islaamic Shari'ah has been disregarded in favor of them except in what they call 'personal matters'."
– "Al-Irshaad ila'Saheeh Al-' Atiqaad ", Vol. 1/ 74

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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 Shari'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 Allah."– "Shubu'haat Howl As-Sunnah Wa-Risaalat Al-Hukm bi'Ghayr ma'Anzal-Allah", Pg. 64

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Imaam Ibn Jareer At-Tabaree said:

"He ta'ala says, whoever conceals the Hukm of Allah, 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 Allah 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 Akbar upon anyone who does what they did.)
– "Tafseer Al-Tabaree" Vol. 4/ 592