From “Al-Tamĥiid li-Qawaaˆid Al-Tawĥiid” by Abu Ath-Thanaa’ Maĥmuud ibn Zayd Al-Laamishiyy Al-Ĥanafiyy Al-Maaturiidiyy:
The Sunni Muslims said regarding the one among the believers (in Islam) who commits a great sin:
- This will be kufr (even in cases of small sins) in the following circumstances:
- If he judged it as permitted (and it is commonly known among scholars and lay people that it is forbidden and he is not like a new Muslim who has never heard of it being forbidden).
- If he did it in scorn to the one who forbade it (e. Aļļaah).
- If he did it for the purpose of rebelling against Aļļaah.
In these above cases he has committed kufr (i.e. become a non-Muslim – and must come back to Islam by uttering the creedal statement with the intention to clear himself of that kufr and come back to Islam).
- However, if he did a great sin due to (such things as) lust, laziness, anger, pride, or disdain (e.g. anger, pride or disdain towards other Muslims – NOT towards something holy, like the rules of Islam, or Angels, or Prophets, because that would be kufr), while also:
- fearing that Aļļaah might punish him for the sin, and yet
- hoping for Aļļaah’s mercy and forgiveness,
then he is called a sinful believer.
The judgment for this person is that if he repents (meeting all of the conditions of complete repentance), then he will be forgiven. However, if he dies before repenting, then it depends on what Aļļaah has willed for him: Aļļaah may forgive him by His grace and mercy, or accept the intercession of a prophet or waliyy among His pious worshippers, or He may torture him for his crime before entering him into Paradise.
(Note that the above position is in opposition to the khawaarijites and muˆtazilites. Both of these sects claimed that the one that commits a great sin, such as adultery, goes to Hell forever if he does not repent. The khawaarijites claimed this because they believe all sins, or great sins at least, to be kufr. The muˆtazilities claimed that committing a great sin puts one in a state between belief and kufr, but that this state means that one goes to Hell forever.)
Sunni Answers 
Salam Alaykum dear Shaykh,
I heard a certain person who is Mutazili-minded in his beliefs mentioning that there is a Hadeeth to the effect that the one who commits adultery or drinks is not a believer while he is committing this act, and that this is one of the evidences for saying that the one who commits an enormity cannot be considered a believer. Is there any response to this from the Sunni ‘Ulaamaa?
wa3alaykumussalaam,, yes, the meaning of this and similar hadiths is that such a person is behaving like a non-believer, or that his belief is incomplete. It’s like when you say in English, “this is not art” when you see a badly made piece of art, i.e. it is incomplete art. The wording is very strong for the purpose of strong rebuke/admonition. This is because of other hadiths, such as the hadiith of Abu Dharr, which states that the person who says, “there is no god but Allaah” (and believes all that it entails of belief in the Oneness of Allaah, the prophethood of Muhammad, etc.) will enter Paradise (eventually), “even if he does adultery and steals”.