Imaan an-Nawawi said in "Kitaab ur-Riddah" (The Book of Apostasy) regarding kufr, "It is the cutting off of Islaam. This occurs sometimes by a statement which constitutes kufr, and sometimes by an act. And the actions which necessitate kufr (that expel from the religion) are those which are performed deliberately, and mocking the religion is clear [in this regard], such as prostrating to an idol or to the sun, or throwing the Qur'an into filth, and the magic which involves worshipping the sun and other such acts." [Rawdat ut-Taalibeen (7/284-283)]

There are certain factors which nullify Islam. Not all acts of kuf (disbelief) will do so, but the following are such grievous acts, that they change the status of a Muslim (believer) to a disbeliever.

1.	Calling on other than Allah such as the Prophets or saints, the dead or alive but absent. Allah the Exalted says: "And invoke not besides Allâh, any that will neither profit you, nor hurt you, but if (in case) you did so, you shall certainly be one of the Zâlimûn (polytheists and wrong-doers)." [10:106] The Prophet  also said in this respect: "Whoever dies while calling on an (so-called) associate-god besides Allah will enter Hell." [Bukhari]
2.	Dislike and unwillingness of the heart to accept the Oneness of Allah and to seek help of Him Alone as opposed to the delight when calling on a Prophet or a pious man, the dead or alive but absent, Allah the Exalted says: "And when Allâh Alone is mentioned, the hearts of those who believe not in the Hereafter are filled with disgust (from the Oneness of Allâh (swt) and when those (whom they obey or worship) besides Him [like all false deities other than Allâh, it may be a Messenger like 'Iesa (Jesus) ­ son of Maryam (Mary), 'Uzair (Ezra), an angel, a pious man, a jinn, or any other creature even idols, graves of religious people, saints, priests, monks, etc.] are mentioned, behold, they rejoice!" [39:45] This verse also condemns those who hate monotheists and call them Wahabi since Wahabism emphasizes monotheism.
3.	Slaughtering in the name of a Prophet or a pious man (saint). Allah the Exalted says: "Therefore turn in prayer to your Lord and sacrifice (to Him only)." [108:2] The Prophet said: "Allah has cursed those who slaughter in the name of other than Allah." (Muslim)
4.	Vowing to any creation with a view to seeking access to him or worshipping him. Vows should be made exclusively to Allah. Allah the Exalted said: (Remember) when the wife of 'Imrân said: "O my Lord! I have vowed to You what (the child that) is in my womb to be dedicated for Your services (free from all worldly work; to serve Your Place of worship), so accept this, from me. Verily, You are the All-Hearer, the All-Knowing." [3:35]
5.	Circumambulation of graves as an act of worship. It is to be performed for Allah alone in the Holy Ka'bah. Allah the Exalted says: "Then let them complete the prescribed duties (Manâsik of Hajj) for them, and perform their vows, and circumambulate the Ancient House (the Ka'bah at Makkah)." [22:29]
6.	Depending on or putting trust in other than Allah. Allah the Exalted says: And Mûsa (Moses) said: "O my people! If you have believed in Allâh, then put your trust in Him if you are Muslims (those who submit to Allâh's Will)." [10:84]
7.	Bowing or prostration before a king or great personalities, living or dead, with an intention of worshipping them. But if one is ignorant, his case is different. Bowing and prostration are acts of worship for Allah alone.
8.	Denying any of the pillars of Islam: Prayer, Hajj, Zakat etc. or the denial of the pillars of Faith (Iman) is also a nullifier of Islam. Denial of anything known to be something of Islam also falls into this type of nullifier.
9.	Hatred for Islam or one of its doctrines - on which Islamic scholars have consensus - like the acts of worship, economy morality or transactions. Allah the Exalted says: "That is because they hate that which Allâh has sent down (this Qur'ân and Islâmic laws, etc.), so He has made their deeds fruitless." [47:9]
10.	Joking about the Qur'an or something relevant to the Sunnah (authentic Hadith) or any Islamic ruling agreed upon by the scholars. Allah the Exalted says: If you ask them (about this), they declare: "We were only talking idly and joking." Say: "Was it at Allâh (swt), and His Ayât (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. If We pardon some of you, We will punish others amongst you because they were Mujrimûn (disbelievers, polytheists, sinners, criminals, etc.). [9:65,66]
11.	Denying some part of the Qur'an or Sunnah amounts to apostasy if it is with knowledge and not with dubiety.
12.	Reviling Allah or the Prophet or cursing the religion or criticizing Allah or the Prophet - all of them amount to blasphemy.
13.	Denying to Allah any of His Supreme Names, Attributes or Actions proved by the Qur'an and Sunnah, provided it is not due to ignorance or interpretation.
14.	Nonbelief in any of the Messengers of Allah: The Messenger (Muhammad SAW) believes in what has been sent down to him from his Lord, and (so do) the believers. Each one believes in Allâh, His Angels, His Books, and His Messengers. They say, "We make no distinction between one another of His Messengers" - and they say, "We hear, and we obey. (We seek) Your Forgiveness, our Lord, and to You is the return (of all)." [2:285]
15.	Judging by other than what Allah has revealed on the assumption that the rule of Islam is unfit or non-Islamic rule is permissible. Allah the Exalted says: "...And whosoever does not judge by what Allâh has revealed, such are the Kâfirûn (i.e. disbelievers - of a lesser degree as they do not act on Allâh's Laws)." [5:44]
16.	Referring to any authority other than Islam for judgement or being dissatisfied with the decision of Islam. Allah the Exalted says: "But no, by your Lord, they can have no Faith, until they make you (O Muhammad SAW) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission." [4:65]
17.	Recognizing other than Allah as source of legislation such as parliaments, democracy etc. The Noble Qur'an says: "Or have they partners with Allâh (false gods), who have instituted for them a religion which Allâh has not allowed. And had it not been for a decisive Word (gone forth already), the matter would have been judged between them. And verily, for the Zâlimûn (polytheists and wrong-doers), there is a painful torment." [42:21]
18.	Forbidding what Allah has made Halal (lawful) or permitting what He has made Haram (unlawful) as adultery, usury or liquor is made lawful absolutely. Making Halal or Haram is an act of Allah alone. The Qur'an says: "Those who eat Ribâ (usury) will not stand (on the Day of Resurrection) except like the standing of a person beaten by Shaitân (Satan) leading him to insanity. That is because they say: "Trading is only like Ribâ (usury)," whereas Allâh has permitted trading and forbidden Ribâ (usury). So whosoever receives an admonition from his Lord and stops eating Ribâ (usury) shall not be punished for the past; his case is for Allâh (to judge); but whoever returns [to Ribâ (usury)], such are the dwellers of the Fire - they will abide therein." [2:275]
19.	Belief in man-made destructive ideologies such as atheistic communism, Jewish masonry, Marxian socialism, secularism or nationalism which prefers a non-Muslim Arab to a Muslim non-Arab. Allah the Exalted says: "And whoever seeks a religion other than Islâm, it will never be accepted of him, and in the Hereafter he will be one of the losers." [3:85]
20.	Changing or abandoning Islam for any other religion. Allah the Exalted says: "...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." [2:217] "Whoever apostatizes from Islam should be killed." (Bukhari)
21.	Assisting the Jews, Christians or the communists against Muslims and cooperating with them. Allah the Exalted says: "Let not the believers take the disbelievers as Auliyâ (supporters, helpers, etc.) instead of the believers, and whoever does that will never be helped by Allâh in any way, except if you indeed fear a danger from them. And Allâh warns you against Himself (His Punishment), and to Allâh is the final return." [3:28]
22.	Not considering the communists or infidels who deny Allah's existence or the Jews and Christians who disbelieve in Muhammad . Allah the Exalted says: "Verily, those who disbelieve (in the religion of Islâm, the Qur'ân and Prophet Muhammad (Peace be upon him)) from among the people of the Scripture (Jews and Christians) and Al-Mushrikûn will abide in the Fire of Hell. They are the worst of creatures." [98:6]
23.	The belief of some Sufis that there is no dichotomy between Allah and the universe (Monism) and that everything is Allah (Pantheism). Their leader Al-Hallaj who was condemned by the scholars and accordingly executed centruies ago said: Allah is I and I am Allah.
24.	Separating the [Islamic] state from the religion and saying that there is no politics in Islam. Such a belief denies the Qur'an and the Sunnah and the life of the Prophet .
25.	Some Sufis believe that Allah has handed over the keys of the affairs of the universe to some of the Auliya (friends of Allah) whom they call Aqtab. This is a Shirk in the Actions of Allah. "To Him belong the keys of the heavens and the earth. And those who disbelieve in the Ayât (proofs, evidences, verses, signs, revelations, etc.) of Allâh, such are they who will be the losers." [39:63]

These invalidators are just like the nullifiers of ablution (Wudu). If a Muslim does any of them, he has to renew his Faith by Taubah (repentance [demonstrated by actions] and abandonment of such a nullifier coupled with returning the rights of the oppressed or if that is not possible, an act to equally recompensate for it). If one remains in this state of kuf and dies before he repents, all his deeds shall be nullified and Hell will be his abode in the Hereafter. Allah the Exalted says:

And indeed it has been revealed to you (O Muhammad SAW), as it was to those (Allâh's Messengers) before you: "If you join others in worship with Allâh, (then) surely (all) your deeds will be in vain, and you will certainly be among the losers." [39:65]

Hadith - Sahih Bukhari 8.800B, Narrated Ikrima from Ibn Abbas, r.a.

Allah's Apostle said, "When a slave (of Allah) commits illegal sexual intercourse, he is not a believer at the time of committing it; and if he steals, he is not a believer at the time of stealing; and if he drinks an alcoholic drink, when he is not a believer at the time of drinking it; and he is not a believer when he commits a murder." 'Ikrima said: I asked Ibn Abbas, "How is faith taken away from him?" He said, Like this," by clasping his hands and then separating them, and added, "But if he repents, faith returns to him like this," by clasping his hands again.

The Prophet also taught us to say the following:

"O Allah! We take refuge with You from any Shirk that we know and seek Your forgiveness for any Shirk that we would not know." (Ahmad)

Related Links:  Knowledge  |  Doubtful Matters  |  Shirk

Action Items for the uttaqun:

• Educate yourself about shirk in its many forms and do not engage in it.

• Do not turn your Muslim brother over to the non-Muslims or contribute to it in any way. Take your complaint first to Allah. Then deal with the Muslim leaders to consult with Islamic Law to address issues between Muslims.

• Be sincere in your Taubah (prayer of repentance) and remember that the Taubah is not sincere nor complete if you do not turn away from the evil deed and then take actions of reparation with the offended Muslim.

Remember... Allah, subhana watala, sees everything we do!