Who is kafir and who is Muslim? People often confuse the difference between kafir asli (originally kafir) and kafir murtad (apostate) who was a Muslim but he negated his belief. Let us classify the types of Kafir below:
1. Kafir Asli - originally kafir
An example of kuffar asli are the: Hindus, Sikhs, Christians, Jews or atheists i.e. a disbeliever in Islam. So they could believe in God but not in The Messenger Muhammad (صلى الله عليه وسلم).
There are three types of Kafir Asli:
(a) Mushriks - from the people of the book i.e. Jews & Christians
(b) Mushriks - who are not people of the book, e.g. Hindus, Sikhs, Buddhists, Zoroastrians etc.
(c) Atheists - such as those who believe in science, self-worshippers, women-worshippers, money-worshippers etc.
Therefore the Kafir is: 'The one who does not believe in Islam and the Prophet Muhammad (صلى الله عليه وسلم).'
2. Kafir Murtad (apostate)
The kafir Murtad a person who was a Muslim once but committed one or more of the actions below:
(a) Al-Juhoud - To deny any obligation known by necessity
If a person denies any action that is known from Islam by necessity, such as to deny fasting, Hajj, prayer, Shari'ah, Jihad he will become Jaahid (the denier), who indulges in Juhoud.
(b) Al-Istihlaal - To permit what Allah (سبحانه و تعالى) forbids or forbids what Allah permits
Istihlaal is to permit what Allah forbids or vice versa. An example of this is the one who permits fornication or forbids. The one who commits Istihlaal is called Al-Mustahil (legislator).
(c) Al-Naaqid - Negations of the Deen
There are ten negations of the Deen (Nawaaqid ul Islam) and if a person violates any one of them he will become murtad.
Nawaaqid ul Islam - the negations of Islam
1. Shirk - Associating someone or something to Allah
Shirk is to associate anyone or anything to Allah (سبحانه و تعالى) such as to say Jesus is the son of God, Tony Blair is the legislator, Ruling by Kufr law (non-Islamic law) etc.
Whether one denies it or not or whether he legalises it or not; anyone who commits Shirk Akbar he will go out of the Millah (Islam) after it is explained to him by an 'Alim. Allah (سبحانه و تعالى) says:
'Verily, Allah forgives not (the sin of) setting up partners (in worship) with him, but he forgives whom he wills sins other than that, and whoever sets up partners in worship with Allah, has indeed strayed far away.' [EMQ 4: 116 & 48]
2. To take an Intercessor between you and Allah
For example, to go to the grave of a dead person and make him an intercessor for whom, the action is kufr and will take him outside of Islam.
Another example is to seek such help from any so-called priest. In the time when the 'Ulama were alive you can ask them for du'a, but when they die, even from the Nabi (صلى الله عليه وسلم), it is not allowed, the Messenger Muhammad (صلى الله عليه وسلم) said: 'Oh Allah! Do not make my grave a place of worship.' Rather when we go to the grave we say: 'Peace to the people of the grave, you are the ones who left first and we are following.' You can ask the one alive for du'a which is mubaah, but if you believe he has power, then this too is Shirk!
3. Whoever does not declare the kafir (non-Muslims) as kafir or doubts that they are kafir or says their religion is correct
There are some moderate 'Muslims' who believe we should not call the Non-Muslims kafir (even though Allah does!) but rather 'Potential Muslims.' Whosoever doubts or believes the Kuffar are not kafir, they become kafir.
At-Takfeer (declaring a person to be a Kafir) is part Usul ud-Deen (one of the Foundations of the Deen) and we are obliged to defend this right of Allah (سبحانه و تعالى). Only Allah (سبحانه و تعالى) has the right to legislate and no one can claim to do so.
4. The one who thinks any Shari'ah (law and order) is similar or better than the Shari'ah of Islam is a kafir
An example of this is the one who believes British law is as good as, equal to as or even better than the law of Islam!
Lord Nazir Ahmed is a good example of this. He left the fold of Islam (an apostate) by declaring that Muslims must obey the British law and Shari'ah law.
5. Whoever hates any of the Shari'ah rules (Hukm Shar'ee) is a kafir
For example, those who hate to pray, to fast, to wear hijaab, to call people to Islam, to work collectively in a group to establish the Islamic state etc.
Allah (سبحانه و تعالى) says: '...that is because they dislike what Allah has revealed.' [EMQ 47: 9]
6. Whoever ridicules any of the Deen of Islam, its rewards or its punishments
Allah (سبحانه و تعالى) says: The hypocrites fear lest a Sûrah (chapter of the Qur'ân) should be revealed about them, showing them what is in their hearts. Say: "(Go ahead and) mock! But certainly Allâh will bring to light all that you fear." (9: 64)
If you ask them (about this), they declare: "We were only talking idly and joking." Say: "Was it at Allâh (سبحانه و تعالى), and His Ayât (proofs, evidences, verses, lessons, signs, revelations, etc.) and His Messenger (صلى الله عليه وسلم) that you were mocking?" (9: 65)
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 Mujrimeen (disbelievers, polytheists, sinners, criminals, etc.) (9: 66)
7. As-Sihr - anyone involving themselves in magic
For example, to communicate with the Jinn, to go to the magician or ask for help from the magician.
8. Alliance with the kuffar and supporting them against the Muslims
Allah (سبحانه و تعالى) says: 'O you who believe! Take not the Jews and the Christians as Awliya' (friends, protectors, helpers) they are but awliya' of each other. And if any amongst you takes them (as awliya') then surely, he is one of them. Verily, Allah guides not those people who are the Zaalimoun (i.e. Kuffar).' [EMQ 5:51]
9. Whoever believes they are not obliged to follow the messenger Muhammad (صلى الله عليه وسلم) or believes that they can leave Islam and go to another Deen is a kafir
10. Whoever turns their back to the Deen of Islam
This can be done by not studying the Deen nor acting upon it. If someone says 'so and so is haram' we must go and check it. Imam Abu Hanifah (ra) was walking on the bank and he saw a child walking on the bank and he was almost falling in, Abu Hanifah said: 'be careful you may fall in' and in response the child said: 'you too, and if you make a mistake many others will also fall in!' Imam Abu Hanifah went home and reviewed all his Ahkaams and said: 'Do not think that what I say today may not be changed tomorrow if I find a hadith' etc…
Allah (سبحانه و تعالى) says: 'Who is more oppressor than the one who knows the Hukm and turns his back? Verily he is Mujrim (i.e. a kaafir).' [EMQ 32:22]