Many people today deny that the violent
 and fanatical Islamic State or ISIL/ISIS is actually Islamic. Appalled 
by its horrendous crimes against humanity, political and religious 
leaders and members of the public alike have been scrambling to claim 
that “the Islamic State isn’t really Islamic.” Muslims who deny ISIL’s 
members as “real Muslims” are engaging in what is called in Islam takfir, the
 act of one Muslim accusing another Muslim of apostasy or being an 
“infidel.” Those Muslims who make such accusations are called takfiri, as described in the Wiki article on “Takfir“:
The latter paragraph describes the Islamic State (ISIL/ISIS) perfectly, as does the next:In principle the only group authorised to declare a member of an Abrahamic religion a kafir (“infidel”) is the ulema, and this is only done once all the prescribed legal precautions have been taken. However, a growing number of splinter Wahhabist/Salafist groups, classified by some scholars as Salafi-Takfiris, have split from the orthodox method of establishing takfir through the processes of the Sharia law. They have reserved the right to declare apostasy against any Muslim, in addition to non-Muslims.…Takfiris…condone acts of violence as legitimate methods of achieving religious or political goals…. A takfiri’s mission is to re-create the Caliphate according to a literal interpretation of the Qur’an.
Takfiris believe in Islam strictly according to their interpretation of Muhammad’s and his companions’ actions and statements, and do not accept any deviation from their path; they reject any reform or change to their interpretation of religion as it was revealed in the time of the prophet. Those who change their religion from Islam to any other way of life, or deny any of the fundamental foundations of Islam, or who worship, follow or obey anything other than Islam, become those upon whom the takfiris declare the “takfir”, calling them apostates from Islam and so no longer Muslim.
Will anyone declaring the Islamic State is “not really Islamic” stand before these takfiris and accuse them of apostasy? Will you sentence them all to death?
According to at least one source (Trevor Stanley), the precedent “for the declaration of takfir against a leader'”came from Medieval Islamic scholar Taqi al-Din Ibn Taymiyyah, who issued a famous fatwa declaring jihad against invading Mongols. This was not because they were invading but because they were apostates, apostasy from Islam being punishable by death.Like jihadis, takfiri groups advocate armed struggle against the secular regime…
Moreover, suicide bombing or other violent act that brings about one’s own death is a legitimate practice of takfiris such as the Islamic State members:
Takfiris believe that one who deliberately kills himself whilst attempting to kill enemies is a martyr (shahid) and therefore goes straight to heaven.
So much for “peaceful Islam” and the “greater jihad,” which represents mere “inner struggle.” Takfiri Islamists cannot live peacefully with secularism and nonbelief. They must
 fight it – and violently, engaging in “lesser jihad” or “struggle 
against those who do not believe in the Islamic God (Allah) and do not 
acknowledge the submission to Muslims, and so is often translated as 
‘Holy War’…”
The takfiris‘ violence thus is 
not a “defense,” except that they feel secularism or nonbelief is an 
offense. Hence, all it takes for these fanatics to be violent is the 
mere existence of nonbelievers.
Being a takfiri (or jihadi) Muslim, therefore, means eternal declaration of war against nonbelievers, regardless of whether or not the latter have done anything overtly to the ummah or global Muslim community.
Thus, the “blowback” excuse proffered by
 “progressive liberals” and “leftists” for Islamist aggression is 
fallacious. The only “provocation” these violent fanatics require is 
non-adherence to their literal, fundamentalist brand of Islam, which is 
why they are currently engaging in wholesale genocide not only of 
non-Muslims but also of other Muslims, whom these takfiris consider to be kuffar (infidels/apostates) and munafiqun or “hypocrites.”
Who is a ‘real Muslim?’
If the Islamic State is not really 
Islamic, we encounter the problem of defining who is a “real Muslim.” It
 is widely claimed there are at least 1.5 billion Muslims in the world, a
 figure held up often by Muslim devotees as a show of strength and for 
purposes of intimidation. However, over the years I have been told 
repeatedly by many Muslims and ex-Muslims that MILLIONS of 
human beings forced to call themselves “Muslim” would like to leave 
Islam but they cannot, under fear of punishment, including possibly the 
death penalty for apostasy. In any event, we can remove millions of 
potential apostates from that figure of 1.5 billion.
Some Muslims also claim the 
fundamentalist Saudis do not represent “real” Islam and that Saudi 
Arabia is not “really” a Muslim country, even though its citizenship is 
100% Muslim. If the Saudis are not “real” Muslims, then we likewise must
 remove 30 million people from the 1.5 billion figure tossed around of 
supposed Muslims in the world.
Next come all others influenced by 
Saudi-style Wahhabism and Salafism, such as the Kuwaitis, Yemenis, UAE 
citizens and many others globally – that’s hundreds of millions more, 
potentially most of the Sunnis, in fact. Remove them from the global 
Muslim total.
How about the Shia Muslims as found in Syria, Iraq, Iran and other places? That would be possibly 200 million people removed from the Muslim count, since they are not “real” Muslims, according to the Sunnis.
The Ahmadis? Remove their millions from 
the count. Ditto with Boko Haram, al Qaeda, Jamaat ul-Fuqra, al Shabaab,
 the Islamic State and thousands of other heretical or extremist groups,
 totaling millions more.
By the time we come up with “real” 
Muslims according to everyone’s standards, there are not so many left in
 the world, far less than 1.5 billion.
By these standards, we cannot say that 
Islam is the second largest religion in the world, since Buddhists and 
Hindus would dwarf these ten to few hundred million “real” Muslims.
In any event, declaring all of these Muslims to be apostates is the act of takfir,
 and one must be prepared to take responsibility for this declaration, 
which is not supposed to be made lightly or by lay persons and which can
 bring with it the death penalty.
Further Reading
http://en.wikipedia.org/wiki/TakfiriWhat is the ummah?
The Truth About Islam
Tom Holland: We must not deny the religious roots of Islamic State
