In contrast to present-day Christianity, the credal system of which is based on a purely spiritual and ethical summons to man and the scope of which does not extend beyond the propagation of religion and the attempt to guide mankind, Islam is a system that refuses to confine itself to mere religious rituals.
The summons to purposive activity and struggle, the need to expand and disseminate the message of monotheism, the elaboration of laws and ordinances for both the material and spiritual life of man, as well as the direct participation of the Most Noble Messenger, peace and blessings be upon him and his family, in various battles - all this indicates that the ideational system of Islam aims at the establishment of a government that with its liberating ideals will permit men to rediscover themselves and choose a truly human form of existence. Further, the government that Islam intends to establish will defend the religion of monotheism with the necessary decisiveness, ward off any aggression against the lands of Islam, and implement God's laws with care and precision.
Such a government, on account of its answerability before God's laws, will never be ready to compromise those laws, however slightly, in the face of pressure and hostility from the enemies of Islam, whatever form that hostility may take; it will never ignore God's commandments nor abandon their implementation.
In general, once religious leadership is separated from the ruling institution and religion remains indifferent to the question of political rule, contenting itself with preaching and admonishing the masses, religion will have no guarantor in society. Even if people are made aware of the teachings of religion by the efforts of scholars and thinkers and try to implement those teachings in their lives, the ruling classes will attempt by various means to prevent the implementation of measures that secure human happiness and especially of the divine commandments that might threaten their hegemony. They will even go beyond that, implementing their own carefully calculated plans to shore up their rule and protect their interests, both in the short term and in the long term.
If therefore religion regards its teachings as the source of salvation and happiness of society, it must give thought to the system of rule, propose a specific system of governance equipped with all the necessary laws and ordinances. Only then will it be able to establish religion in society and clear the way for God's religion to advance.
Both in Islam and in the monotheistic religions that preceded it, particular attention was paid to the establishment of a suitable system of government, an entirely logical concern, for the founders of different schools of religious thought were unwilling to abandon the fruits of their efforts to the vicissitudes of history.
Islamic government - i.e., the administration of the ummah on the basis of Islamic law began with the migration of the Prophet, peace and blessings be upon him and his family, and his arrival in Madinah; it was then that the system of Islamic government began to take shape.
From the very first day that the Messenger of God laid the foundations of Islamic monotheism, despite the hostility of the corrupt and misguided polytheists whom he had left behind in Makkah, and began expanding the power of Islam in Madinah in all its aspects, political, economic, geographical and cultural, he entrusted the administration of certain matters to responsible and capable elements so that they might contribute to the advancement of the community.
Throughout the battles and wars that took place in order to remove the obstacles that stood in the way of spreading the truth and to establish justice, new lands would come under the control of the Muslims. The Prophet would immediately, appoint in each of these lands a governor and a judge as well as a teacher whose task it was to teach religion to the people. Protection was also extended to the non-Muslims inhabiting these territories and whatever humane cultural values they cherished.
The Qur'an recognizes the Prophet to have had the functions of ruler (hakim) and judge (qadi), for it addresses him as follows: "Judge among them according to what your Lord has sent down to you, and follow not their vain desires." (5:48)
The prophets were indeed the founders of divine government on earth, and they constituted the principal resource for the establishment of a righteous government that would serve the broad masses of the people.
The Qur'an assigns rulership not only to the Prophet of Islam but also to Yusuf (Joseph): "When Yusuf reached maturity We bestowed on him rulership and knowledge; thus do We reward the doers of good." (12:22)
The Qur'an addresses Dawud (David) as follows: "O Dawud, We bestowed upon you Our viceregency upon earth, so that you might rule justly among God's creation, not following your vain desires for they would lead you astray from God's path." (38:25)
The Islamic laws concerning fixed penalties and the payment of blood money, as well as many other topics in jurisprudence, count as the executive pillars of the Islamic system of government that was founded by the Prophet.
The governmental function of the Prophet had another important dimension, that of creating a suitable environment for the strengthening of the Islamic summons, expounding the divine laws and ordinances for mankind, and inculcating in men the concepts of God's Book so that they might attain the lofty goals that Islam had prescribed.
The Qur'an says the following with respect to this aspect of the Prophet's task: "God it is Who raised a great Prophet from among the unlettered Arabs in order that he might recite to them the verses of God's revelation and cleanse them from the pollution of ignorance and evil morals, and teach them the shari'ah of God's Book and divine wisdom, even though they were previously in the pit of ignorance and misguidance." (62:2)
The Prophet was thus the ruler of Islamic society in addition to being a guide and a promulgator of God's ordinances. Whoever wished to be his successor ought, then, to have combined in himself these same two dimensions rulership of society and the spiritual guidance of the ummah. In addition, he had to be the guardian of the credal based of Islam and its ordinances, protecting them from change or distortion and resisting decisively the assaults of unbelief, skepticism and misguidance; one able to solve problems arising from any kind of deviation, and to confront any aggression of the Islamic lands by outsiders. Only thus could the continuity and preservation of Islam be assured amid the myriad dangers It faced.
The best method of preserving the rights of the individual and society is to have a government of the righteous, and the most righteous form of government is without doubt the government of the inerrant (ma'sum), which alone makes it possible to hope for the preservation of the rights of all man. A government headed by one chosen by God Is in reality the government of God, and It is only this type of government that makes it possible for man to preserve his true personality, nobility and dignity, and to attain all his rights. Respect for the dignity of man and the establishment of justice are among the fundamental principles of such a government. Impious and arbitrary rulers may often pay lip service to human rights and claim to be the defenders of the dignity of the individual and society, but in practice they drag man's honor in the mud, and their only accomplishment is the enthronement of discrimination and Injustice.
There can then be no doubt as to the importance of a just and virtuous government and the efforts of the prophets to establish such, nor conversely of the damage caused by impious rulers who are unconcerned with the ultimate destiny of society and with enabling men to obtain their rights.
The one who wishes to assume religious leadership and undertake the guidance of the masses as the successor of the Prophet, must bear affinity to him with respect to knowledge, deeds, and manner of thought. He must also have special moral qualities and spiritual attributes, be divinely protected from sin, and be fully cognizant of the truths of religion; only then will he be able to solve whatever problem arises on the basis of truth, justice, and the shari'ah. Islam cannot accept that rule over society and the protection of human dignity should be entrusted to the first person who chances along.
The Noble Qur'an cites the superior strength and capacity of Talut (Saul) as a reason for his being chosen as a fit leader of his people: "He is more fitted to rule because God has chosen him and bestowed on him a surfeit of knowledge and power." (2:247)
In just the same way that the Most Noble Messenger, peace and blessings be upon him and his family, exercised two offices, the one who wishes to succeed him must also have his two essential qualities: an inner dimension of connectedness with God which is bestowed by God Himself in His grace, and an outer dimension of leadership and rule. These two are inseparable, and leadership of the ummah cannot be based on one of them alone; political and social leadership must go together with spiritual guidance. The Imam has both spiritual and legislative authority, and is thus able to perpetuate the correct mode of administering human affairs that was established by the prophets.
When the fifth Imam was commenting on the belief prevalent at the Saqifah that the two aspects of the matter can be separated from each other, he cited this verse in which God bestowed on the progeny of Ibrahim both spiritual guidance and leadership and the administration of society:
"Are they envious of what We in our generosity have bestowed on the progeny of Ibrahim? We sent the Book and wisdom to the family of Ibrahim, and gave them too kingship and rule." (4:52)
Then the Imam added: "How is it that they accept the combination of the two aspects in the case of the children of Ibrahim, but reject it for the Household of the Prophet?"
 al-'Ayyashi, al-Tafsir, Vol. I, p.247.