وقال الذين كفروا لولا نـزل عليه القرآن جملة واحدة كذلك لنثبت به (25:32) بالانجليزي

(25:32) The disbelievers say, "Why has not the entire Qur'an been sent down to this man all at nce?" *44 -Well, this has been done to impress it deeply on your mind, *45 and (for the same object) We have sent it down piecemeal by degrees.

Tafseer Abu-al-A'la Maududi

Page 362 English Traditional

*44) As the disbelievers of Makkah considered this objection to be very strong, they repeated it over and over again. The Qur'an also has cited it with its answer at several places; for instance, see E.N.'s 101-106 of Surah An-Naml and E.N. 119 of Bani Isra'il. Their question implied: "Had the Qur'an been really the Word of Allah, it would have been sent as a complete book aII at once; for Allah has the knowledge of everything and every human affair. Thus it is obvious that nothing is being sent down from above; but this man himself fabricates all its themes or gets these from other people or other books. "
*45) Another translation can be: "So that by it We may strengthen your heart and imbue it with courage." The words are comprehensive and imply both the meanings. This concise sentence contains the following explanation why the Qur'an was revealed piecemeal by degrees:
(1) So that the Holy Prophet may commit it to memory perfectly and recite it to his people, who are illiterate, rather than present it in a written form.
(2) So that its teachings and messages may be impressed deeply on the minds.
(3) So that the way of life it teaches, may be followed with complete conviction, which would not be possible if all the Commandments and the whole system of life had been sent down all at once.
(4) So that the hearts of the Prophet and his followers may be imbued with courage during the conflict between the Truth and falsehood. This required that the Divine Guidance and messages of encouragement should be revealed as and when needed according to the practical situation. Obviously, this could not have been possible if these had been sent down all at once. This also showed that Allah had not left His Messenger alone amidst persecution to counter all sorts of resistance and opposition after appointing him to the mission, but He Himself was watching the struggle with concern and guiding His Prophet through every difficulty by direct communion in every critical situation.