I came across this helpful list on a website; it helps us understand when Islam considers someone a kaafir or an apostate. The writer, Muhammed A. Hafeez, warns that the list is not intended to be comprehensive, however 'it is general and every person should be forewarned against the matters mentioned therein. Kufr should be recognised and acknowledged by the Muslims in general so that they know how to protect themselves from its dangers.'
We must however remember never to be hasty in labelling one who claims to be Muslim as a kaafir (disbeliever) or murtadd (apostate) without clear and irrefutable evidence. An assignation or specific ruling of kufr (disbelief) for such a person should generally be left to the scholars of Islam. Kufr (disbelief) is of two types: action ('amaly) and belief ('itiqaady ), some of which may be termed "kufr doona kufr" that is, a degree of kufr less than the total kufr which does not necessarily put one outside of Islam although such a person's Islam may be doubtful or in grave danger. This is the explanation of Qur'an 5:44 by the companion Ibn Abbaas (radiallaahu 'anhu) collected by Ibn Jareer and quoted by Ibn Katheer on the authority of 'Ataa.
Ibn Umar (radiallaahu 'anhu) reports that the Prophet (sallallaahu 'alaihi wa sallam) said "Any person who called his brother 'O Unbeliever' (has in fact done an act by which this kufr) would return to one of them. If it were so as he (the accuser) asserted, (the kufr of the accused would be confirmed, but if the kufr was not true) then it returned to him (the man who called his brother Muslim a kaafir)." Muslim.
There are matters which may indeed indicate or even necessitate that a person be a disbeliever (kaafir) such as defiling the Qur'an or cursing the Prophet (sallallaahu 'alaihi wa sallam) regardless of what the person espouses and it is only Allah who knows the unseen. As humans we can only judge or make determinations from what is outwardly evident.
1. To disavow Allah's Lordship (Ruboobiyah) or Deity and singular right to be worshipped (Uloohiyah) [Qur'an 4:48 & 116, 5:72, 39:2-3, 51:56] or the message of any of the Messengers of Allah (Tawheed) [Qur'an 10:35, 16:36] or to claim that any Messenger or Prophet (Rasool or Nabiy) came after the Prophet Muhammad (sallallaahu 'alaihi wa sallam). [Qur'an 6:93, 6:21, 7:37, 11:18-19, 18:15, 29:68, 33:40, 39:32, 61:8]
2. To deny or reject any of Allah's Attributes or Names (Asmaa was Sifaat) i.e. Living, Knowing, Hearing, Seeing, Merciful, having a face and hands (though we know not in what manner yet certainly not like His creation), or to add to them that which is not mentioned in the Qur'an or authentic Sunnah. i.e. Belief that Allah is not above His creation and that He is everywhere or in everything or to attribute any quality that is Allah's alone to man [Qur'an 4:48, 6:18 & 61, 10:3, 68 - 69, 20:5, 35:10, 42:11, 72:26 - 27, Ch. 112]
3. Setting up intermediaries between oneself and Allah, making supplication to them, asking their intercession, and placing one's trust in them. Calling upon the dead, asking them for help, or offering them gifts or sacrifices is all shirk. [Qur'an 2:165 & 255, 5:72, 30:52, 39:44]. To give any of the creation that which is due to Allah alone or to be pleased with being given worship besides Allah in any manner [Qur'an 21:29] thus making or becoming a taaghoot (pl. tawaagheet).
4. To curse, abuse the Name of Allah, to deny or to revile Him or any of His Messengers or Prophets or Angels. [Qur'an 2:285, 4:136, 6:10, 57:19]
5. To deny, stop, or reject any Fard (obligatory) duty of the Sharia (Divine Law) i.e. Salaah, Zakaah, Saum, Hajj, Kindness to parents, or Jihad. [Qur'an 4:50, 64-65, 80] or to turn away from the religion by not learning or practising its precepts. [Qur'an 32:22]
6. To deem or to make permissible (halaal) what is clearly forbidden (haraam) - ie adultery, fornication, drinking alcohol, taking drugs, theft, murder, sorcery, magic; or to make what is halaal haraam - ie to eat meat or seafood, marry plural wives, women to veil, etc. [Qur'an 2:102, 174-176,4:69, 6:157]
7. To deny or reject any chapter, verse, or letter from the Book of Allah (Al-Qur'an) or to purposely give it deviant interpretations [Qur'an 3:7, 6:21] i.e. not derived from the Qur'an itself, the authentic Sunnah of Muhammad (sallallaahu 'alaihi wa sallam) [Qur'an 6:153, 16:64] or according to established methods of tafseer. Abu Hurairah (radiallaahu 'anhu) reported that the Rasool (sallallaahu 'alaihi wa sallam) said, "Don't pursue that which you have no knowledge of." Ahmed. [Qur'an 4:59, 10:68-70].
8. To openly show disdain, scorn, contempt for, or treat with levity the Deen of Islam or its tenets, obligatory injunctions or it's traditions. [Qur'an 4:140] To ridicule and deride them or to treat the Qur'an as a piece of garbage or to trample or abuse it as an insult. [Qur'an 4:83, 140, 6:4-5, 7:50-51, 9:63-66, 20:124, 41:26 27]
9. To disbelieve in the Resurrection, Punishment, or Goodly reward on the Day of Reckoning or that punishment and reward are only abstract, symbolic or spiritual. [Qur'an , 56:1-2, 67:8-10, 69]
10. To say that the Righteous (Awliyaa') are above the Prophets ('alaihimus salaam) or that some of the Righteous are exempt from acts of worship prescribed by Islam according to the Sunnah of Muhammad (sallallaahu 'alaihi wa sallam). [Qur'an 3:85, 8:34, 10:62-63]
11. To rule or judge by other than that which Allah revealed. This is of different types and the ruling differs depending on the beliefs and actions. Whoever rules by other than what Allah has revealed viewing it or believing it to be superior or better than Allah's sharia is considered a disbeliever by all the Muslims.
Likewise, the one who substitutes the sharia with man-made laws and views that as permissible, even if he says that to rule by the sharia is better, is a disbeliever because he has made halaal what Allah has made haraam. If one rules as described following whims or in opposition to someone (i.e. not viewing it as equal, a substitute or superior as described above) he is considered a major sinner. [2:120, 3:85, 4:115, 5:3, 44, 49, 50, 33:36, 45:18, 58:22]. [explained in detail in Fataawa Ibn Baz, Vol. 1 Rulings on 'Aqeedah, pg. 991]
12. To support or aid the polytheists (mushrikoon) against the Muslims. [3:28. 118, 4:51, 5:57]