An Islamic State is an ideological state. Keeping in view its specific ideology, its citizens can be classified into two categories:  
    1) Those who become its citizens after accepting its ideological basis.  
    2) Those who assume its citizenship as a consequence of a treaty.  
 
The First Category  
    The Qur’an has specified three conditions for persons eligible for this category:  
    1) They should profess faith in the oneness of Allah and the prophethood of Muhammad (sws) and give up every belief and deed against Islam.  
    2) As testimony of their faith, they should offer prayers according to the way prescribed by the Prophet (sws).  
    3) For the functioning of the state system, they should pay zakat to the public treasury (baytu’l mal).  
    The Qur’an  says:  

If they repent [from all un-Islamic beliefs and deeds], establish regular prayers, and pay zakat, they are your brethren in religion. (9:11)  

    Whoever fulfils these three conditions, will be considered a Muslim in the eyes of the law and will be granted this type of citizenship in an Islamic State. As far as his rights and duties in a state are concerned, there will be no difference between him and those who are the founders of the state. The Qur’an  has used the word fa ikhwanukum fi’l din (they are your brothers in religion) to convey this meaning. From the word al-din, Islam is implied and by the words fa ikhwanukum, those who professed faith in the Prophet (sws) in the early stages and those who laid the foundations of the Islamic state in Madinah have been addressed and told that people who fulfil these conditions are equal to them and will have the same political and collective rights.  
    The Prophet (sws) has explained this Qur’anic directive in the following words:  

I have been ordained to wage war^* with these people until they testify to the oneness of Allah and the prophethood of Muhammad, establish regular prayers and pay zakat. If they accept these conditions, their lives and wealth shall be given protection except if they are deprived from this protection on the grounds of some offence they may commit. As far as their inner account is concerned, it rests with Allah. (Muslim: Kitabu’l Iman)  

        A similar statement is attributed to the Caliph Abu Bakr, while he was launching an attack against those who were desisting to pay zakat:  

The Prophet waged war on three conditions: on testifying to the oneness of Allah, on the establishment of regular prayers and on the payment of zakat and the Almighty has said: `Therefore, if they repent, establish regular prayers and pay zakat, spare their lives'. By God! I shall neither ask for more nor less. (Ahkamu’l Qur’an, Jassas, Vol 3, Pg 82)  

The Second Category  
    The Qur’an  has alluded to this type of citizenship in the words `with whom you have concluded a treaty’ (8:56). The Jews of Madinah are implied here. This treaty was concluded with them by the Prophet (sws) when he became the ruler of Madinah after migration. Historians refer to it as the Mithaq-i-Madinah. Later on, similar treaties were concluded by the Muslims with other nations as well. Such treaties, of course, can be concluded upon different conditions depending upon the circumstances. In the Mithaq-i-Madinah, it was clearly stated that after accepting Muhammad (sws) as the ruler of Madinah, the Jews and the Muslims would be one nation. Their rights will be the same as the rights of the Muslim citizens of Madinah:  

The Jews, according to this treaty stand accepted as one nation with the Muslims. As far as their religion is concerned, the Jews will remain on theirs’ and the Muslims and their allies on theirs’. (al-sirah al-nabawiyyah, Ibn Hisham, Vol 2, Pg 107)  

    All non-Muslim citizens of an Islamic State belong to this category. 
     

^*. The words qital (war) must not cause any misconception in the hadith quoted. The opponents in this case were the Ismaelites, the nation towards whom the Prophet (sws) was directly assigned. For them the law was either to accept faith or face destruction. Citizenship meant the fulfilment of the three conditions stated in verse eleven of Surah Tawbah quoted above. Therefore, the Companions of the Prophet (sws) waged war against 'them if they declined to fulfil even one of them.