Apostasy refers to when a Muslim becomes a disbeliever by saying a clear statement to that effect, or by uttering words which imply that (i.e., which imply kufr or disbelief), or he does something that implies that (i.e., an action which implies kufr or disbelief). Linguistically, returning, as indicated in the words of Allaah, the Exalted: "And do not (Taraddu) return back." Quran Surah al-Maidah 5: 21. The Arabic word for Apostasy is Riddah.
That is, do not revert to infidelity, while Riddah in judicial terminology is apostasy after Islam. Allaah, the Exalted, says: "…. And anyone of you who reverts (Yartadid) from his Deen and dies as an infidel, it is those whose good deeds shall be vain in this world and in the world to come. These are inmates of the Fire, wherein they shall live forever." Quran Surah al-Baqarah 2: 217
Our Prophetﷺtold us that this religion would reach all places on this earth. It was narrated that Tameem al-Daari said: “I heard the Messenger of Allaahﷺsay: ‘This matter (i.e., Islam) will certainly reach everywhere that night and day reach, and Allaah will not leave any house or tent [i.e., all dwellings, in towns and in the desert], but Allaah will cause this religion to enter it, and some people will be honoured because of it [by converting] and others will be humiliated because of it [for refusing to embrace it], and they will be ruled by the Muslims, an honour which Allaah will bestow on Islam and a humiliation which He will inflict on kufr (disbelief).” Musnad Ahmad, 16344; classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, 3
The matters which constitute apostasy are divided into four categories:
(a) Apostasy in beliefs, such as associating others with Allaah, denying Him, or denying an attribute which is proven to be one of His attributes, or by affirming that Allaah has a son. Whoever believes that is an apostate and a disbeliever.
(b) Apostasy in words, such as insulting Allaah or the Messenger (peace and blessings of Allaah be upon him).
(c) Apostasy in actions, such as throwing the Qur’an into a filthy place, because doing that shows disrespect towards the words of Allaah, so it is a sign that one does not believe. Other such actions include prostrating to an idol or to the sun or moon.
(d) Apostasy by omission, such as not doing any of the rituals of Islam, or turning away from following it altogether.