Ibn Hazm (Rahimahullaah) said “Whoever rules by the Gospel in issues where
there is no text from revelation in the Sharee’ah of Islaam, then he is a
Kaafir Mushrik outside of Islaam.” (Ihkaam al-Ahkaam Fee Usool al-Ahkaam
5/153)

Also, Ibn Taymeeyah (Rahimahullaah) said,

“Its known by necessity from the religion of the Muslims and by the agreement of all the Muslims, that he
who permits the following of a religion other than the religion of Islaam
or following a law other than the Sharee’ah of Muhammad (Sallallaahu
'Alayhi wa Sallam), then he is a Kaafir.” (Majmoo' al-Fataawaa 28/524)

And he similarly says, “The Sharee'ah revealed from Allaah (Ta’aalaa),
which is the Qur’aan and Sunnah that Allaah sent His Messenger with, no
one from the creation is allowed to leave it. And no one leaves it but a
Kaafir.” (Majmoo' al-Fataawaa 11/262)

And he says, “Whenever a person legalizes what is illegal by consensus or
illegalizes what is legal by consensus or replaces the Sharee'ah that is
agreed upon by consensus, then he is a Kaafir by the agreement of the
scholars of Fiqh.” (Majmoo' al-Fataawaa 3/267)

Ibn Katheer (Rahimahullaah) says “So whoever leaves the clearcut Law
revealed to Muhammad bin ‘Abdillaah the final of the Prophets, and refers
to other sources for judgment, such as abrogated laws, then he is a
Kaafir.” (al-Bidaayah wan-Nihaayah 13/119)

‘Abdil-Lateef bin ‘Abdir-Rahmaan Aal ash-Shaykh says, “Whoever refers to
something other than the Qur’aan and Sunnah for judgment, after it being
clarified to him, then he is a Kaafir. Allaah (Ta’aalaa) Says, {And
whoever rules by other than what Allaah Revealed, then they are
disbelievers.} and Allaah Says, {Do they hope for other than Allaah…}”
(ad-Durar as-Saneeyah 8/241)

‘Abdullaah bin Humayd says, “Whoever legislates a law, which is general
and enforced upon the people, opposing the law of Allaah, then he has left
Islaam as a Kaafir.” (Ahammiyat al-Jihaad pg. 196)

Muhammad bin Ibraaheem Aal ash-Shaykh says, “Verily from the Major clear
cut Kufr is giving to the accursed manmade laws the position of that which
the faithful spirit descended upon the heart of Muhammad that he should be
a warner in the clear Arabic tongue, judging by it between the nations,
and referring back to it when there is a disagreement, in contradiction
to, and obstinate rejection of, Allaah, the Mighty and Exalted’s saying:
{If you differ in anything among yourselves, refer it to Allaah and His
Messenger, if you do believe in Allaah and the Last Day. That is best and
more suitable for final determination.} (an-Nisaa’ 4:59) Allaah the Most
Perfect and High has negated the belief of those who do not make the
Prophet the judge in that about which they differ – strengthening the
negation by repeating it and swearing upon it, He says: {But no, by your
Lord, they can have no belief, until they make you a judge in all disputes
between them, and find no resistance in themselves against your judgments,
an
d instead submit wholeheartedly.} (an-Nisaa’ 4:65)” (Risaalat Tahkeem
al-Qawaaneen)

He also mentioned in the same treatise that from the greatest types of
Major Kufr of this nature, is what the modern apostates have fallen into,
he said:

“Fifth: This is the greater, more comprehensive and obvious form of
stubborn rejection to the Sharee’ah and haughty rejection of its rulings
and showing dissent of Allaah and His Messenger, and challenging the
courts of Islaamic Law, in its setting-up, maintenance, provisions,
foundations and its branches, forms, types, its judgment and compulsion,
references and sources. So just as the Sharee’ah courts have references
and sources upon which they depend – all depending upon Allaah’s Book and
the Sunnah of His Messenger – then in the same way the law-courts of the
manmade laws have their sources: laws patched together from many different
law-systems, like French laws, American laws, British laws, etc. and from
the systems of innovators who claim allegiance to the Sharee’ah, etc. So
these law-courts are now present in many Muslim lands, complete and
established, with open doors and the people flock thereto in throngs;
their judges give judgments going against the judgments of the Book and
Sunnah – according to
their own laws and making these pronouncements binding and agreeing
thereto and making them unalterable, so what Kufr is greater than this,
and what can be a greater contradiction of the testification that Muhammad
is the Messenger of Allaah?”

Ash-Shaykh ash-Shinqeetee says in ‘Adwaa’ al-Bayaan “The strange thing is
from those who rule by other than the Sharee’ah of Allaah, and then
thereafter claims Islaam. Like Allaah (Ta’aalaa) Says, {Do you not see
those who claim they believe in what was revealed to you and what was
revealed before you? They wish to refer to false gods for judgment
although they were commanded to reject them? Satan wishes to deviate them
very far.} And He Says, {Whoever rules by other than what Allaah Revealed,
then they are the disbelieves.} And He Says, {Should I seek someone other
than Allaah as a judge although it is He who revealed to you the book that
is detailed? Those who were given the book by Us know that it was revealed
with truth by your Lord, so do not be from the doubters.}” (3/441)

And he said (Rahimahullaah) “Those who follow the laws of the legislators
instead of the Sharee’ah of Allaah, are Mushriks associating partners with
Allaah.”  (4/82-83)

Shaykh Ahmad Shaakir says, “These manmade laws were forced upon the
Muslims by the enemies of Islaam, who are public with their enmity. These
laws, in reality, are another religion that they gave to the Muslims as a
replacement to their pure and heavenly religion (of Islaam). This is
because they obligated upon the Muslims obedience to this law, and planted
in their hearts sanctification of it and fanaticism to it, to the point
that many tongues and pens utter ‘sanctification of the law’, ‘holiness of
the judgment’, ‘sacredness of the court’. They use many similar
descriptions regarding manmade law which they refuse to describe the
Islaamic Law by and the opinions of the Islaamic scholars of Fiqh.
Instead, when they describe it, they call it reactionary, barbaric, clergy
law, law of the jungle, etc.” (‘Umdat at-Tafseer 3/214)

He also says, “The matter of these manmade laws is clear as the clarity of
the sun, it is open and clearcut Kufr which there is no secret or flattery
about. There is no excuse for anyone who ascribes to Islaam, whoever he
may be, to act upon it, or submit to it, or recognize it. So each person
should be careful for himself and take note of himself.” (‘Umdat
at-Tafseer 4/174)

Shaykh Muhammad Haamid al-Faqqee says, “What we can summarize from the
words of the Salaf is that Taaghoot is all that deviates the slave and
turns him from worshipping Allaah, being sincere in religion to Allaah,
and obeying Allaah and His Messenger.
This is whether that person or thing is a devil from the humans or Jinn, or if that thing is a tree or rock,
etc.

What is also included amongst Taaghoot, without doubt, is ruling by
manmade laws which are foreign to Islaam in affairs of life, marriage,
divorce, and ownership, to invalidate the Laws of Allaah, like
establishment of the prescribed punishments, prohibition of interest,
adultery, fornication, intoxicants, etc. These things which the manmade
laws decided to legalize and defend by means of its makers and
implementers. So, the laws themselves are Taaghoot, as well as those who
legislate and make these laws.” (Footnotes to Fath al-Majeed)