This was a question which the disbelievers had asked right from the time of Muhammad (pbuh) itself. Look at what the Qur'an has to say: 'Those who reject Faith say: 'Why is not the Qur'an revealed to him all at once? Thus (is it revealed), that We may strengthen thy heart thereby, and We have rehearsed it to thee in slow, well-arranged stages, gradually.' (25:32)
'(It is) a Qur'an which We have divided (into parts from time to time), in order that thou mightiest recite it to men at intervals : We have revealed it by stages.' (17:106)
It becomes clear to us from these verses as to why the Qur'an was not revealed in a completed book form. The Qur'an remains a religious scripture that is to provide guidance for all mankind which is to come right up to the Last Day. It is never a mere collection of legal decrees like the Torah. It is a book that particularly demonstrated the method by which a society may be cleansed of its decadence solely through transformation in thought and belief; in morality and values. Indeed, confronting us, today, is the picture of a society that had actually transformed itself according to the stages in which the Qur'an came to be revealed. It is, in fact, with the aid of this picture that we can seek to understand the complete meaning of each one of its verses. If the Qur'an was revealed as a complete whole, such an important method of appreciation of its contents would have been impossible. Furthermore, it would not have been possible to verify the statement that the Qur'an is a wholly practicable book. As a book that was revealed in stages to a growing community - a community that evolved in step with the said stages - we can say with certainty that it is practicable.
The benefits that have accrued from the Qur'an being revealed in stages can be summarized thus:
1. It is difficult to eradicate, in one stroke, the vices and immorality which had prevailed in society since a very long time. It is only in stages that they can be stopped. To alienate the people from their wanton ways gradually, before establishing a permanent set of rules and regulations, will be seen to be more practical. Towards this end, revelation that comes in stages remains the ideal facility indeed.
2. The reception of divine commands in a fashion in which the doubts that arose in the minds of the people as also their immediate problems were addressed and solved, proved a favourable factor in the effective transformation of those who were addressed.
3. In the event that the revelation was made all at once, it would have become imperative to implement all the laws and decrees that were prescribed in it in a single day. It is not necessary here, to reassert the difficulty of such a procedure. Through revelation in stages, this difficulty is put to rest and it becomes possible to gradually raise a community that has been cleansed of all immorality.
4. That the revelation came to him at various intervals was a matter of great consolation and strengthening of faith for the Prophet.
5. For the illiterate Muhammad (e), the revelation which came in stages afforded a facility for the study, and memorization, of the Qur'an. It, thus, set the stage for the complete freedom from forgetfulness and mistakes.
6. Through the process of gradual revelation it becomes possible for the followers of the Prophet to memorize and study deeply the issues contained in the Qur'an and to regulate their lives according to its mandate.