Question:
Is it permissible to declare a specific individual a disbeliever, and did the Salaf declare specific individuals as disbelievers in the past?
Answer:
Praise be to Allah, Lord of the Worlds, and may peace and blessings be upon our Prophet Muhammad, and upon all his family and companions. I bear witness that there is no deity worthy of worship except Allah alone, without partner, and I bear witness that Muhammad is His servant and Messenger.
To proceed:
Regarding declaring a specific individual a disbeliever when they commit an act that necessitates disbelief—such as contradicting a definitive text or deeming a forbidden matter as permissible—there is no harm in doing so, provided that (as previously mentioned) they actually commit what necessitates disbelief. Otherwise, the Prophet (peace and blessings of Allah be upon him and his family) said, as recorded in Al-Sahih: “Whoever says to his brother, ‘O disbeliever,’ it will apply to him if it is true; otherwise, it will return upon the one who said it.”
As for cursing a specific individual who is a sinner among the Muslims, the correct view is that it is not permissible. This is because the Prophet (peace and blessings of Allah be upon him and his family) said to Umar—when he spoke to Nu’ayman, as recorded in Sahih al-Bukhari: “May Allah disgrace you, how often you are brought [for drinking alcohol]!”—the Prophet (peace and blessings of Allah be upon him and his family) said: “Do not aid Satan against your brother.”
However, regarding declaring a specific individual a disbeliever when they commit what necessitates disbelief, Imam al-Shafi’i (may Allah the Exalted have mercy on him) said to Hafs al-Fard: “Indeed, he is a disbeliever,” because he claimed that the Qur’an is created.