If a person was a Muslim, then he apostatised from Islam and became an atheist, he is an apostate, and there is no confusion about that. The one who apostatises from Islam to another religion is not regarded as a follower of that religion and does not come under the same rulings as followers of that religion. Rather he is an apostate, like the atheist mentioned above, and is to be treated as such. He is to be asked to repent and come back to Islam; if he repents and comes back (all well and good), otherwise he is to be executed because of his disbelief (kufr).
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) said:
If anyone apostatises and becomes a Jew or a Christian, we do not accept that, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever changes his religion, execute him.” Narrated by al-Bukhaari (2854). In other words: whoever changes from being a Muslim, we should execute him.
Ash-Sharh al-Mumti‘ ‘ala Zaad al-Mustaqni‘, 11/306
Based on that, if a person was Muslim then he became a Christian or a Jew, meat slaughtered by him is not permissible because he is not a kitaabi (one of the People of the Book); rather he is an apostate whose meat cannot be eaten. If such a person is a woman, it is not permissible to marry her, for the same reason. These people cannot be dhimmi or mu‘aahad or musta’man (one who is granted protection and security by the Muslims), because these statuses can only be granted to one who was originally a kaafir, not to an apostate. The one who apostatised from Islam has no choice but to return to his religion or to be executed as a kaafir, if he persists in his apostasy.
Shaykh Saalih al-Fawzaan (may Allah preserve him) said:
In shar‘i terminology, the apostate (murtadd) is the one who disbelieved willingly after having been a Muslim, by words, beliefs, doubts or actions.
There are rulings concerning the apostate in this world and in the hereafter.
With regard to this world, it was explained by the Messenger (blessings and peace of Allah be upon him) who said: “Whoever changes his religion, execute him.” The scholars are unanimously agreed on that and on what results from it of his wife being separated from him and his being prevented from disposing of his wealth before he is executed.
With regard to the hereafter, that is stated by Allah, may He be exalted, in the verse (interpretation of the meaning): “And whosoever of you turns back from his religion and dies as a disbeliever, then his deeds will be lost in this life and in the Hereafter, and they will be the dwellers of the Fire. They will abide therein forever” [al-Baqarah 2:217]. Apostasy occurs when one commits any of the actions that nullify Islam, whether that is done in earnest or in jest or in mockery. Allah, may He be exalted, says (interpretation of the meaning): “If you ask them (about this), they declare: "We were only talking idly and joking. Say: Was it at Allah ( ), and His Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) and His Messenger (SAW) that you were mocking? Make no excuse; you have disbelieved after you had believed” [at-Tawbah 9:65].
Al-Mulakhkhas al-Fiqhi, 2/565, 566
See also the answer to question no. 20060 for further information.
We should point out that the hadd punishment for apostasy – like all other hadd punishments – cannot be carried out except by the caliph or his deputy, according to scholarly consensus. It is not permissible for individuals to carry out the hadd punishments themselves.
With regard to the way he should be dealt with, that is the same as any other disbeliever: he should be called to Islam with kindness, reminding him of his past when he was a Muslim, and that he must inevitably feel the difference between his life in Islam and his life outside of it. Help may be sought in doing that from those who know his language and come from his country, so that it will be easier for him to understand. Giving gifts also has a good impact on people’s hearts, so give him things that he needs, by means of which you will be able to reach his heart. If he has some difficulties in some worldly matters you can help him with them and make things easier for him. All of that will make him think again about what he has done, and perhaps he will come back to Islam before he dies, and that will be recorded among your good deeds.
All of that has to do with interacting with him. With regard to the ruling, we have stated above that he is not like one who was originally a kaafir. It is not permissible for a Muslim to inherit from an apostate or for him to inherit from a Muslim, and the apostate woman cannot marry a Muslim. If a Muslim woman is married to the apostate, her marriage contract is rendered invalid. And there are other rulings. But we should differentiate between the rulings and how we treat the individual.
And Allah knows best.