Tell me if it is wrong to call a Muslim a non-Muslim if he turns his back on Islam.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Describing a Muslim by Kufr (disbelief) is a dangerous matter which should not be taken lightly.

The Prophet said: “If a man says to his Muslim brother: “O, Kafir! (Disbeliever)”, then one of the two will have Kufr drawn on him.” [Al-Bukhaari]

In another narration, the Prophet said: “If a person says to his brother “O, Kaafir! (Disbeliever)”, then it is like killing him.”

This is clear evidence that describing a specific person with disbelief (Kufr) is not an easy matter as it can lead to bad consequences in this life and in the Hereafter.

In fact, the Muslim does not become a non-Muslim unless he says, or does, or believes in something that the Quran or the Sunnah considers major Kufr (disbelief). Besides, there should be conditions of Kufr and absence of impediments such as compulsion and the like. So, one should beware of describing a Muslim by Kufr.

Indeed, whether or not someone is Kaafir is determined only by Allah and his Prophet, sallallaahu ‘alayhi wa sallam, and not by the likes and inclinations of the people.

Allah knows best.