It is known that al-Kufr al-Bawaah (manifest, clear kufr) is an open, outward matter, such as when he abolishes the teachings of Islaam, or we see him for example, destroying mosques, or he fights the people of the mosques (i.e. those who frequent them), or he abolishes the [Sharee'ah] law courts, or he abolishes the religious lessons, for example, or we see him burning the copies of the Qur'aan, or that he orders for them to be burnt, and he promotes, assists the books of misguidance, the books of the Christians, and whatever resembles them, and he spreads them and makes reading them to be binding, or we see him erecting those things that are worshipped besides Allaah, such as idols and the likes. This is considered manifest, clear kufr.

As for the [types of] matters in which ijtihaad can enter into, then we alluded to one of these types last night. And this is what the majority of the rulers (wullaat) are upon, from that which is called "judgement by the secular laws" (hukman bil-qawaaneen), such as these laws, overwhelmingly, the affair pertaining to them is that they consider benefit (maslahah) in them, but they did not abolish the legislation (shar') with a complete abolition, such that they do not judge with anything from it at all.

Since Allaah said, "And whoever does not judge by what Allaah has revealed they are the disbelievers" (al-Maa'idah 5:44), so the likes of these, when they have this angle of approach, then we do not speak of their kufr, but we consider them to be in error, in this ijtihaad which involves changing something from the legislation, even if it was by the path of ijtihaad. So for example, their permitting of zinaa [i.e. in action, not as a matter of belief], when it is with the consent of both parties, and like their abandonment, or the abolition of the hudood, the punishment for stealing, or the punishment for false slander, or the punishment for drinking alcohol, or permitting alcohol [i.e. in action, not as a matter of belief], and announcing the selling of alcohol, and whatever resembles that.

There is no doubt, that this is a great sin, however there could be, for example, excuses for them, those in which they consider themselves to be justified (i.e. excused in that). So for example, they excuse themselves from this by saying that in their land they have those people who are not Muslim, and that being severe upon them will make them flee. So when they have an angle of approach, then Allaah will reckon them, but, in any case, there is no doubt that if we judged by the shar' (legislation), and implemented its teachings, there would be sufficiency in this and much good.

Source: Cassette: Sharh Lum'uat il-I'tiqaad, (No. 7), Tasjeelaat at-Taqwaa, Riyaadh

See this too…

Shaykh Abdul-Azeez Ar-Raajihee was asked:

This person asks about the Sharee'ah ruling concerning the ruler who rules by the French Secular Laws alongside the knowledge that he claims Islaam, prayers, fasts and makes hajj. So what is to be said about him?

The Answer: When he believes in [their] permissibility (i'taqada al-jawaaz), when he believes that judgement by the French [secular] laws is permissible, then he is a kaafir. When he believes that it is permissible for him [to do that].

As for when he does not believe this, or he has a doubt (shubhah), then it is necessary for the proof to be established against him.

And some of the people of knowledge have held that when he alters the religion (ghayyara ad-deen) in all of the affairs of the state, then he is a kaafir, because he has changed (baddala) the religion, and al-Haafidh Ibn Katheer (rahimahullaah) has gone to this [view] in his tafseer, and also Shaykh Muhammad bin Ibraaheem (rahimahullaah) in his treatise "Tahkeem ul-Qawaaneen". So when he changes the religion, the whole of it, from head to heel (i.e. top to bottom), in all of the affairs of the state, in everything - not in part of it - then he is a kaafir, because he has altered the religion.

And then some others have said that it is necessary for the proof to be established against him, for he could be ignorant, or have some doubt (shubhah). Our respected Shaykh, Abdul-Azeez Bin Baaz (may Allaah's mercy upon him), chose this view. See the link for the article above and similar ones:

(http://www.themadkhalis.com/md/articles/tlafh-shaykh-abdul-azeez-ar-raajihee-on-ruling-by-the-secular-laws-and-altering-the-entire-religion-deen.cfm).