Sheikh al-Islam Mustafa Sabri, the last Sheikhul Islam (the chief judge) in the Ottoman Empire, May Allah be merciful to him, says as follows:

When a state deviates from Islam and says: „following the laws of religion is not the matter of the state, rather only a matter of the people", then this is seperation of religion from the state. In such a case applies, that the the state has deviated from Islam (Apostasy, riddah, ارتداد). If the people agree with such a state or the state legislates laws in Parliament representing the people (through elections), both the state and the people deviate from Islam (i.e. become Kafir / Non-Muslim). In this case the following verse applies to the state as well as to the people: 

„(...) Those who do not rule according to what Allah revealed (Sharia, divine law) are the disbelivers (Kuffar)." (The noble Quran 5:44)

A political regime that advocated the principles of laicism (seperation of religion and state), is regarded as such a regime that opposes the islamic state (not ISIS, Islamic rulership in general).

As a result, first the state apostates from Islam, and then those who follow the state, each of them fall into apostasy. Just as those who participate in this goverment fall into apostasy, so will the masses of people who obey this state will all become apostates (i.e. Kuffar / Non-Muslim). 

No other event could be more terrible that this sudden and immediate apostasy of the masses.

[Mauqif al-'Aql wal-'Ìlm wal-'Âlim mir-Rabbi'l 'Âlamīn, Position of reason, knowledge and scholars with the Lord of the Worlds, 4/286]