Short answer
No. In Sunni belief, a Muslim who commits a major sin such as zina, theft or drinking is a sinful believer, not a disbeliever, unless he believes the sin is halal or mocks the religion. He is under Allah's will: Allah may forgive him or punish him, but he will not stay in Hell forever.
We do not declare anyone who prays toward the qibla a disbeliever because of a sin, as long as he does not deem it lawful. This is stated in al-Fiqh al-Akbar and the Tahawi creed and is the position of all Sunni schools of creed.
The four schools agree on this.
Explanation
The Prophet ﷺ said Jibril told him that whoever of his ummah dies without associating anything with Allah will enter Paradise. Abu Dharr asked, "Even if he commits zina and steals?" He said, "Even if he commits zina and steals." Entering Paradise may come after punishment, or Allah may forgive from the start, but the sin does not cancel faith.
The Quran calls two groups of Muslims who fight each other "believers" (49:9). It calls the murderer and the victim's family "brothers" in the law of retribution (2:178). Murder is among the worst sins, yet faith remains.
What about hadith such as "The adulterer is not a believer while he commits adultery"? Sunni scholars explain these as denying complete faith, not faith itself. In that moment his faith is weakened and he is far from the believer he should be.
Early in Islam the Kharijites declared major sinners disbelievers, and the Mu'tazila placed them in a state between faith and disbelief. Sunni scholars rejected both. The line is crossed only when someone denies something clearly known to be part of Islam, calls a clear haram halal, or mocks Allah, His Messenger or the religion.
This belief gives hope, not permission. Major sins are serious and need repentance. It also means no Muslim should call another a kafir over sins; see What does kufr mean?
The evidence
Quran, Surah Al-Hujuraat 49:9
وَإِن طَآئِفَتَانِ مِنَ ٱلْمُؤْمِنِينَ ٱقْتَتَلُوا۟ فَأَصْلِحُوا۟ بَيْنَهُمَا ۖ فَإِنۢ بَغَتْ إِحْدَىٰهُمَا عَلَى ٱلْأُخْرَىٰ فَقَٰتِلُوا۟ ٱلَّتِى تَبْغِى حَتَّىٰ تَفِىٓءَ إِلَىٰٓ أَمْرِ ٱللَّهِ ۚ فَإِن فَآءَتْ فَأَصْلِحُوا۟ بَيْنَهُمَا بِٱلْعَدْلِ وَأَقْسِطُوٓا۟ ۖ إِنَّ ٱللَّهَ يُحِبُّ ٱلْمُقْسِطِينَ ﴿٩﴾
And if two factions among the believers should fight, then make settlement between the two. But if one of them oppresses the other, then fight against the one that oppresses until it returns to the ordinance of Allah. And if it returns, then make settlement between them in justice and act justly. Indeed, Allah loves those who act justly.
Muslims who fight each other are still called believers.
Quran, Surah Al-Baqara 2:178
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ كُتِبَ عَلَيْكُمُ ٱلْقِصَاصُ فِى ٱلْقَتْلَى ۖ ٱلْحُرُّ بِٱلْحُرِّ وَٱلْعَبْدُ بِٱلْعَبْدِ وَٱلْأُنثَىٰ بِٱلْأُنثَىٰ ۚ فَمَنْ عُفِىَ لَهُۥ مِنْ أَخِيهِ شَىْءٌۭ فَٱتِّبَاعٌۢ بِٱلْمَعْرُوفِ وَأَدَآءٌ إِلَيْهِ بِإِحْسَٰنٍۢ ۗ ذَٰلِكَ تَخْفِيفٌۭ مِّن رَّبِّكُمْ وَرَحْمَةٌۭ ۗ فَمَنِ ٱعْتَدَىٰ بَعْدَ ذَٰلِكَ فَلَهُۥ عَذَابٌ أَلِيمٌۭ ﴿١٧٨﴾
O you who have believed, prescribed for you is legal retribution for those murdered - the free for the free, the slave for the slave, and the female for the female. But whoever overlooks from his brother anything, then there should be a suitable follow-up and payment to him with good conduct. This is an alleviation from your Lord and a mercy. But whoever transgresses after that will have a painful punishment.
The killer is still called a brother of the victim's family.
Quran, Surah An-Nisaa 4:48
إِنَّ ٱللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِۦ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَآءُ ۚ وَمَن يُشْرِكْ بِٱللَّهِ فَقَدِ ٱفْتَرَىٰٓ إِثْمًا عَظِيمًا ﴿٤٨﴾
Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allah has certainly fabricated a tremendous sin.
Sins less than shirk are under Allah's will to forgive.
Sahih al-Bukhari 1237
Sahih... وَإِنْ زَنَى وَإِنْ سَرَقَ ...
"Even if he commits zina and steals."
Major sins do not stop a believer who dies on tawheed from entering Paradise in the end.
Sahih al-Bukhari 2475
Sahih... لاَ يَزْنِي الزَّانِي حِينَ يَزْنِي وَهْوَ مُؤْمِنٌ ...
"The adulterer is not a believer at the moment he commits adultery."
Explained by Sunni scholars as denying complete faith, not faith itself.
Where the ruling is recorded
- al-Aqida al-Tahawiyya, Section on the people of the qibla. States that no one of the people of the qibla is declared a disbeliever for a sin unless he deems it lawful.
- al-Fiqh al-Akbar (Imam Abu Hanifa), Section on sins and faith. States that a Muslim is not declared a disbeliever for any sin, even a major one, unless he deems it lawful.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.