Takfi̅r basically means to attribute the state of disbelief (kufr) to a person. Kufr, according to a definition given by Ibn Ḥazm, means to deny something that God has made a constituent part of belief, and that such denial may occur by means of the heart only or the tongue only or both of them, or otherwise by a faith-violating act. The above definition tackles kufr in terms of being someone's normative status (Al-kufr Al-Aṣli) that stems from dogma. However, the kufr covered in this article concerns the conversion from belief to disbelief, and hence takfi̅r means to judge or declare a person apparently thought to be a Muslim as a disbeliever.
Due to its sensitive nature and grave consequences, the leading Muslim scholars and Imams have advised caution when dealing with the topic of takfi̅r. Thus, Imam Abu Hanifah is reported to have said, "If an utterance consists ninety-nine percent of disbelief but one percent of belief (Iman), it would still not amount to kufr." Moreover, there were instances, for example, during the reign of the fourth Caliph, ‘Ali b. Abi Talib, where the Kharijites went extreme and made accusations of disbelief against even the leading figures among the Companions. Although the Kharijites were regarded as transgressors, on no occasion did the Caliph declare any of them to be an infidel (kafir); instead he said that a person who seeks the truth but makes an error is never the same as a person who seeks falsehood and then proceeds to commit it. The same cautious attitude was also adopted by the second Caliph, 'Umar b. al-Khaṭṭab. During his time, a person known as Quda̅mah Ibn Maẓ‘u̅n made a statement that wine-drinking was permissible in Islam. However, the Caliph did not declare him a ka̅fir, saying that it was a matter requiring evidence in order to ascertain and establish the facts of the case before any judgment was passed on it.
With regard to the nature of the evidence that needs to be produced, the general rule is that determining the faith or disbelief of a person is to be based entirely on obvious and explicit evidence without any inference of the hidden thoughts and intents of people, which are known to God only. This prerequisite is made clear by the verse where Allah commands: "Say not to those who greet you with peace (as-sala̅m): "you are not a believer". The purport of the above verse is confirmed by the hadith which states: "Whoever prays like us and faces our Qiblah and eats our slaughtered animals is a Muslim and is under Allah's and His Apostle's protection. So do not betray Allah by betraying those who are in His protection."
The warnings featured in many hadiths concerning the prohibition of takfi̅r are indicative of the enormity of such utterances. Thus, in one hadith the Prophet warned: "If anyone says to his brother, 'O disbeliever! Then surely, one of them is as such." The Prophet reiterated the same purport of this hadith in different wording, stating: "Whoever charges another person with disbelief, or calls him an enemy of God, while he is not so, will have the charge redound upon himself." The prohibition in question is not confined to only charging a Muslim with disbelief, but it also extends to cover lesser offensive charges directed at one's moral conduct, such as fisq (disobedience). Thus, in one hadith the Prophet declares: "No man accuses another of transgression (fisq) or disbelief (kufr) without partaking in it himself if the accused is not what the accuser claims he is."
From the bulk of the above proofs, it may be concluded that no one may charge another with disbelief or transgression without clear-cut evidence, and he who commits this violation partakes in the violation himself. The scholars agree that the matter of judging a person as a disbeliever is not to be decided by assumption or deduction whatsoever, and that it is to be decided by evidence that eliminates any shred of doubt, such as auditory or visual evidence, that is, to hear or see a person do a faith-violating act or make a statement of the same. Being a grave sin in Islam, unfounded takfi̅r is a form of expression that is not granted protection under the Sharia. It is also a major offence that is penalized by a deterrent ta‘zi̅r punishment.