The Foundation of Islam is based in two matters: Negation (Nafi) and Affirmation (ithbaat).

And it was said by Imam Muhammed Ibn Abdil Wahab At-Tammimi an-Najdi:

“The foundation and principle of the Deen of Islam is in two matters:-

Firstly: Commanding the worshipping (directing of every type of Ibadah) to Allah (swt) alone without any partner, encouragement upon this, making friendship for its sake, and making Takfeer [1] of whosever leaves it.

Secondly: Abandoning Shirk (joining partners) in Ibadah to Allah (swt) and warning from it, being severely harsh upon this, being hostile for its sake and making Takfeer [2] of whosoever does it” [3]

[1] Takfeer: Declaring someone to be a Kafir, expelling one from the fold of Islam. Making Takfeer of any Muslims is forbidden, and some of the scholars held it to be an act of Kufr on its own, but it is obligatory to do Takfeer of every apostate and Kaffir. Shaykh Abdullah Ibn Muhammed Ibn Abdil Wahab said: “And in summary, it is obligatory upon those who are sincere not to speak about these matters without knowledge or evidences from Allah (swt). And he should beware lest he expels a person from Islam merely because of his own interpretation. And “expelling someone from the fold of Islam”[Takfeer] and entering someone into the fold of Islam is from the greatest matters of the Deen… And indeed Shaytan has misled many in this regard; so there are such people who consider a person to be Muslim but the texts of the Quran, Sunnah, and Ijmaa prove that he is a Kaffir, and then there are others who make Takfeer of those who are actually Muslim according to the Quran, Sunnah, and Ijmaa’.” Refer to “Ad-Duraar As-Saniyyah” (8/217)

[2] Mark this in your heart, O brother of Tawheed ! Unlike the Murj’iah, the Imaam has clarified that doing Takfeer of the apostates is amongst the foundations and the bases of Islam. But what do we see today from the Murj’iah of this era ? We see nothing except a warning against making Takfeer of the apostates. And anyone who does Takfeer of the apostates is branded as a “Takfeeri”, and a “Khaariji”. And it has gone beyond this to such a degree, that if someone calls Bush a “Taghout” and “Kaffir” publicly, he is branded as a “Khawaarij” – since in their opinion, Bush cannot be labelled as a Taghout unless he declares his own legislations to be Islamically Halal, Ya Subhanallah! Let the people open up the books of history and read what the Mushrikeen had branded the Scholars of Najd with – let your eyes find the Mushrikeen calling the Mujaddid with the exact same disgusting terms, “Takfeeri”, and “Khaariji”, etc. And refuge is sought with Allah (swt).

[3] Refer to “Ad-Duraar As-Saniyyah” (2/203)