When in 656 the Muslims acknowledged Ali as their Caliph, and gave him their pledge of allegiance, religious and temporal authority were combined in one person. They thus “endorsed” an arrangement which had been made, as early as 632, by the Messenger of God himself for his succession.
There is no commandment in Islam, major or minor, doctrinal or formal, which has been left to the whims or the wishes or the votes of the masses. The most important political institution in Islam is the caliphate. It is important because the existence of the Muslim community and the survival of Islam depend upon it. It would, therefore, be unthinkable that it would be left to the whims or the wishes or the votes of the street crowds.
Law in Islam is the expression not of man's will but of God's.
After the death of Muhammad, Ali did not have any political power but he was still his successor. Whether or not he had power in his hands, obedience was due to him as the successor of the Prophet of Islam. The only thing that changed, after Ali's election, was that those people who had withheld their obedience to him in the past, now gave it to him, voluntarily.
Of those Muslims who took the oath of loyalty to Ali, there were two groups. Both groups took the oath of loyalty to him but for different reasons. The first group knew him to be the head of the Kingdom of Heaven on Earth; the second group acknowledged him only as the Chief Executive of the government of the Muslims.
The first group knew that Muhammad himself had designated him as the leader of the Muslim umma, and it knew that it was not free to take the oath of loyalty to anyone else. The second group, however, would have given its pledge of loyalty to anyone who would have succeeded in seizing power.
Muhammad was not only a teacher and an ideal leader but was also the pioneer of a new age on earth for all mankind. He opened the gates of a new age, introduced into history a new force called Islam, let loose in the world a new dynamic that can, and does, and will, change human life and transform human relationships. He was the “pioneer” of all those men and women who seek the salvation of all mankind.
The basic aim of Muhammad was to establish the Kingdom of Heaven on Earth, i.e., the Government of Islam. He taught mankind the lesson of Tauheed (the doctrine of the Oneness of the Creator), and he invited it to acknowledge His paramountcy. He promulgated God's laws, enforced them, and created a society the hallmark of which was purity.
In that society there was the reign of justice, knowledge and enlightenment, and he eliminated exploitation, tyranny, ignorance and superstition from it. Islam, the only monotheistic religion that represents a complete socio-economic-political system, is inherently hostile to all secular governments, especially those that adopt alien values which are repugnant to the value-system of Islam.
The establishment of the Kingdom of Heaven on Earth was the first part of the mission of Muhammad. The second part of his mission was to assure its continuity. He did not establish the “Kingdom” only for his own lifetime but for all time, and not only for the Arabs but for all mankind. He, therefore, designated as his successor a man he knew would give continuity to his work. Such a man was Ali ibn Abi Talib, as noted before.