Is it permissible to describe someone as a heretic (zindeeq) without proof? And what is the advice for one who does that?

Linguistically, "zandaqa" (heresy/apostasy) means narrowness or confinement. According to the majority of jurists, it is the outward display of Islam while concealing disbelief, which was initially termed "nifāq" (hypocrisy) in the early days of Islam. However, for the Hanafis and some Shafi'is, it signifies not adhering to any religion, or believing in the eternity of time, or holding the belief that wealth and women are communal.

Labeling a Muslim as a "zindiq" means accusing him of major disbelief (kufr akbar) that expels him from the fold of Islam. This is a serious matter that is impermissible unless the conditions for his excommunication are met and the impediments are absent. Furthermore, such a judgment must come from someone deeply rooted in the Islamic sciences, who is aware of the individual's circumstances, and after the legal proof (al-ḥujjah ash-sharʿiyyah) has been established against him. The matter becomes even more critical if a person declares another to be a disbeliever without legitimate proof and without observing the regulations of excommunicating a specific individual (takfīr al-muʿayyan) according to the Ahl as-Sunnah (definition) wa al-Jamāʿah. Indeed, excommunicating a specific individual before establishing the proof against him and offering him an excuse is impermissible.

Shaykh al-Islam Ibn Taymiyyah said: "No one has the right to declare any Muslim a disbeliever—even if he errs or makes a mistake—until the proof is established against him and the clear path is shown to him. And whoever's faith is established with certainty, it is not removed from him by doubt. Rather, it is only removed after establishing the proof and removing the ambiguity." He also said: "General excommunication, like general threats, must be affirmed in its generality and inclusiveness. However, judging a specific individual as a disbeliever or testifying that he is destined for Hell: this depends on specific evidence, for a judgment depends on the establishment of its conditions and the absence of its impediments."

This issue is serious, and whoever accuses others of "zandaqa" without proof is in peril, for the Messenger of Allah, peace and blessings be upon him, said: "Any man who says to his brother, 'O Kafir,' then it has surely returned to one of them. If it is as he said, then good; otherwise, it has returned to him."