Unfortunately not all men have remained on right path. As Allah has repeatedly said in the Qur'an, an overwhelming majority has deviated from the true faith.
Accordingly, Islam has divided them in various categories, which are given below:
One who believes and affirms that there is only One God, that Muhammad (s.a.w.a.) is His Last and Final Prophet and Messenger, and that there is to be a Day of judgement, is called a Muslim. We may also say that anyone who believes in earlier-mentioned Usul al-Islam (i.e. Tawhid, Nubuwwat and Qiyamat) is a Muslim.
A Muslim who also believes that Ali (a.s.) was the First Imam after the Holy Prophet (s.a.w.a.) by appointment of Allah, is called Shi'a.
A Muslim who believes in divinely-appointed twelve Imams (mentioned earlier) and in 'Adl of Allah, is named Shi'a Ithna 'ashariyah. (Ithna 'Asher = twelve; Ithna 'asheriyah= the believers in twelve Imams; the Twelver’s). We may also Say that anyone who believes in the five usul-e deen is a Shi'a Ithna 'asheriyah.
If a person does not believe in existence or oneness of God, or the prophethood of the Prophet of lslam (s.a.w.a.) or the Day of Judgement, he is called kafir, (unbeliever). Plural: Kuffar (unbelievers).
To make our meaning clearer, we shall call them non-Muslims.
Non-Muslims are divided in two main categories:
1) Mushrik: one who ascribes any partner or colleague to God? Plural Mushrikeen (polytheists.)
2) Ahlul-kitab: (the people of the book) i.e. the Jews and the Christians.
There are some groups which claim to be Muslims, but they are kuffar, and the Shi'a Ithna 'ashari shari'ah treats them such. They are as follows:
3) Anyone who rejects, insults or doubts any of the Zaruriyat al-deen (the things everyone knows as part of Islam). For example, paradise and hell, 5-times daily prayer and many other things. A person rejecting, doubting or insulting such things is kafir, even though he may be reciting kalimah a hundred times a day. (See chapter 6 for explanation and some details.)
4)Ghali ((inordinate); plural: ghulat): Anyone who believes that any of the prophets or Imams was god, or that God was incarnated in him, or that any of the Imams were prophet in short, anyone who assigns to the prophets or Imams a status higher than their true position and status is called Ghali: such a person is kafir even if he claims to be Muslim.
5) Khariji (plural khawarij): Those who separated from 'Ali (a.s.) after the arbitration, claiming that 'Ali (a.s.) had become, God forbid, kafir. Khawarij are kuffar in the eyes of shari'ah.
6) Nasibi (plural Nawasib): anyone who hates any of the twelve Imams or insult him. Nasibi too is kafir. '
7) Anyone who abuses, or uses foul language, about the Prophet (s.a.w.a.), Bibi Fatimah, or any of the Twelve Imams is kafir.
There are certain rules governing all the above-mentioned kuffar:
(1) A mushrik is najis (unclean). As for the people of the Book (i.e. the Jews and the Christians) opinions differ. The reader should consult the 'amaliyyah of his mujtahid.
(2)A non-Muslim man is not allowed to marry a Muslim woman, a non-Muslim woman is not allowed to marry a Muslim man. The only exception is a Christian woman or Jewess marrying a Muslim man in Mut'ah form.
(3) A non-Muslim cannot inherit from a Muslim - no matter how close their relationship may be.
(4) A non-Muslim cannot possess a Muslim Slave.
(5) An animal slaughtered by a non-Muslim is treated as a dead animal; it is not lawful to eat from that animal.
(6) Meat, fat, hide and skin, in possession of a non-Muslim are najis, unless one knows that he has got it from a Muslim.
(7) It is not wajib on Muslims to conduct funeral rites of a non-Muslim.
(8) There is a special rule about the last mentioned category of kafir, i.e. the one who abuses or uses foul language concerning any of the 14 Ma'sumeen. It is wajib (compulsory, obligatory) on any Muslim who can do so, to kill him - without any need of referring the case to the qadi of shari'ah.
There is a particular kind of kafir who is called Murtadd (apostate), i.e. a person who accepted Islam and then renounced it. The position of Murtadd is worse than an original kafir. Some important rules about murtadd are given here.
Murtadd is of two kinds: Murtadd Fitri and Murtadd Milli.
Murtadd Fitri: A person both (or at least one) of whose parents were Muslims at the time of his conception, and then, he/she, on attaining majority, renounces Islam by his own free will.
The rules about Murtadd Fitri differ according to the sex of the offender, i.e. there are different rules for males and females.
A male Murtadd Fitri is najis; his wife becomes haram, for him, and she will observe 'iddah of death; even during his life he loses all rights in his property - after repaying his debts the residue will be distributed among his Muslim heirs. And his repentance is not acceptable in this life; according to well-known ruling of the scholars. But Ayatullah al-Khoui says that obviously his repentance is acceptable in the hereafter and to a certain extent in this life too. Accordingly, if he repents he will become tahir, and if his former wife agrees he may remarry her in a fresh marriage. In short, he will be treated as a Muslim - but even then he shall not regain possession of his property unless the heirs willingly return it to him. Likewise the sentence of death shall not be waived even after repentance.
If a woman becomes Murtadd Fitri, she becomes najis; and haram on her husband, but she does not lose her property nor is she sentenced to death; and her repentance is accepted. If she refuses to repent, she will be imprisoned.
Murtadd Milli: A person who was not born of a Muslim parent: and accepted Islam after attaining majority and later renounced it.
He should be told to repent. If he repents within three days, his repentance will be accepted. Otherwise he would be killed on the fourth day. His Muslim wife will become haram for him, i.e. the marriage will become invalid automatically, and she will have to observe the 'iddah of divorce if the marriage was consummated; but if he repents during her 'iddah period, the marriage will continue without there being any need for a new nikah.
Murtadd Milli is debarred from managing his property (although he remains its owner). If he repents, he will regain its management; otherwise he will be given his essential requirements from it until he dies; after his death, the property will be divided among his Muslim heirs.
(The readers should consult detailed books of jurisprudence for details of the rules about both types of apostates.)
Now we come to a special category of men whom the Qur'an calls Munafiq (hypocrite). Plural = Munafiqun .
Munafiq is a person who professes to accept Islam but in reality does not believe in it.
Such a person is treated as a Muslim in this life: His/her body is tahir; he/she may marry with a Muslim spouse; he/she will inherit from his/her Muslim relatives; his/her life, honor and property will get as much protection in law as that of any other Muslim.
But in the hereafter, he/she will be treated as kafir or even worse. The Qur'an says:
“Surely the hypocrites are in the lowest rank of the fire . . .” (4:145).
Mu'min: (believer): This nomenclature has various usages;
Often the Qur'an addresses the Muslims with the phrase, O you who believe!; Obviously it covers all those who recite the kalimah.
At other times, the Holy Book differentiates between Islam and Iman, using the word, Islam, for verbal profession, and Iman for the firmly-rooted belief:
In Shi'a traditions and the writings of Shi'a scholars, the word, mu'min, is commonly used for a practicing Shi'a Ithna 'ashari.