God, in His grace, never left mankind without a religious guide. That guide may be a prophet; a 'Rasul' or an 'Imam '. The first man, Prophet Adam (a.s.), was made a vicegerent of Allah on this earth, so that he might lead his children on the right path.
Since then, prophets and messengers were sent to all the regions and all the peoples. Allah says in the Qur'an:
"And there never was a people without a warner having lived among them". (Qur'an, 35:24)
In all, there came 124,000 prophets from God. Many of the prophets were sent to one or two villages, some even to one family or one man. Others were sent to a bigger area; still others to a whole tribe. But none of them, before our Holy Prophet, was sent to the whole mankind.
Our Holy Prophet was sent to the whole mankind for up to the end of the world. No other prophet is to come after him. He was, and is, the Last Prophet.
It appears from the history of divine religions that God sent from time to time many 'Shari'ahs' (Divine Laws) which were suitable to that particular era. Prophet Noah (a.s.) brought a Shari'ah which was simple to a great extent. And that Shari'ah was followed by other prophets up to the advent of Prophet Abraham (a.s.). Prophet Abraham (a.s.) was given a Shari'ah which was more elaborate and more comprehensive than the previous one.
The Shari'ah of Prophet Abraham remained in force for the children of Israel up to the time of Prophet Moses (a.s.). When Prophet Moses (p.u h.) was given Torah (Law), it was a really comprehensive and fully detailed Shari'ah, which was followed by all the prophets of Bani Israel till Prophet Jesus (a.s.) came. Prophet Jesus (a.s.) perfected the Shari'ah of Prophet Moses (a.s.) and made adjustments according to the time. The, Shari'ah of Prophet Jesus (a.s.) remained valid till the arrival of the Holy Prophet of Islam (S).
Coming to the other branch of the family of Prophet Abraham (a.s.) we find that the children of Ismael were expected to follow the Shari'ah of Prophet Abraham (a.s.) up to the time of the Holy Prophet of Islam, Prophet Muhammad Al-Mustafa (S). When he came, he abrogated and cancelled all the previous Shari'ahs, and brought the final, most comprehensive and most suitable and moderate Shari'ah of all, which can meet the challenge of the changing trines without any difficulty up to the Day of 'Qiyamah (the Day of Resurrection).
It may be asked: Why the changes in Shari'ahs? Why the gradual revelation? And why the separate 'Ulul-Azm' (Prominent) prophets coming one after another? Well, when a child is born, the parents make some garments for him. And as the child continues to grow, the old clothes are discarded, and new ones made according to the growth of the body of the child, this continuous during his childhood, during his adolescence, during his teenage, till a time comes after 25 or 30 years, when the body reaches its maximum height and attains its full growth.
After that, the size which fits him at that time continues to fit him up to the end of his life. Nobody will suggest that as the child at the age of 25 years is expected to be 5 ft. 6 in. tall, he should be given the clothes of that size on the day of his birth. Nor will anybody think that a young man of 30 years should wear the same clothes which he used to wear when he was 10 years old. Likewise, we may suppose that the humanity was a child in the days of Prophet Adam (a.s.) and Noah (a.s.), which reached its adolescence in the days of Prophet Abraham (a.s.) and continued to grow (mentally, socially and spiritually). Accordingly, Allah continued discarding and abrogating old Shari'ahs and sending new ones according to the social, intellectual and spiritual needs of the times.
This continued up to the time of Prophet Muhammad Al-Mustafa (S). This time may be compared as the age of 25 or 30 years of a man when he reaches the full height and the highest peak of his strength. Now there is no chance that he will outgrow his clothes, and the size of what age remains in force till his last day. When humanity reached that stage, Allah sent the final Shari'ah which was to serve the mankind to the last day of the world. After Muhammad Al-Mustafa (S) there was no need for any Shari'ah; there was no need for any new Prophet or messenger from God. And it was for this reason that he was declared by Allah to be the last of the prophets.
Question: Admitted that the body does not grow in height after about 25 years; but still there appear changes in the body. A person may gain or lose considerable weight, necessitating some changes in the measurement of his clothes. Therefore, now can you say that there will never be any need for a new Shari'ah after Islam?
Answer: Clothes usually do not adjust themselves according to the build of a body. But Islam has a built-in capacity to cover all the possible situations which a man faces during his life-time. In this respect, we may compare it with those sophisticated electronic devices which automatically adjust to the temperature, light, humidity and other relevant factors of the operating time. If you take a good camera, you will find that its lens makes all the necessary adjustments according to the distance and light without any need for you to make those adjustments manually.
Likewise, Islam has all the rules for all the possible situations, and as soon as a given situation changes, the Shari'ah' automatically recognizes the change and another set of rules applicable to the new situation comes into force immediately and automatically.
This flexibility is the unique feature of Islam which is not found in any other religion. And this feature eliminates the need of a new Shari'ah.
Of course, the need for an interpreter of the Qur'an and protector of the Shari'ah will remain forever. But Allah appointed Imams for this purpose, after the Last Prophet. The chain of the Prophethood came to an end and a new system of religious leadership, known as 'Imamat' was introduced. The Holy Prophet said "Bani Israel, prophets were leading them; when a prophet died another prophet succeeded him. But after me there is no prophet, and surely there will be Caliphs".