﴿ وَقَالَ ٱلَّذِينَ كَفَرُوا۟ لَوْلَا نُزِّلَ عَلَيْهِ ٱلْقُرْءَانُ جُمْلَةًۭ وَٰحِدَةًۭ ۚ كَذَٰلِكَ لِنُثَبِّتَ بِهِۦ فُؤَادَكَ ۖ وَرَتَّلْنَٰهُ تَرْتِيلًۭا ﴾

“Now they who are bent on denying the truth are wont to ask. “Why has not the Qur’an been bestowed on him from on high in one single re­velation?” [it has been revealed] in this manner so that We might strengthen thy heart thereby - for We have so arranged its component parts that they form one consistent whole -”

Commentary The sequence of objections by the infidels and polytheists and their answers, which had started in the beginning of the Surah, is continuing. Here in this verse the objection as to why the Qur'an was revealed gradually bit by bit and not in one go is answered. The wisdom behind gradual revelation of the Qur'an as explained in the verse is that it was to keep the heart of the Holy Prophet ﷺ firm and strong. By gradual revelation the Holy Prophet's ﷺ heart was made strong in the following manner: 1. It was made easy for him to remember the Qur'an by heart. If the whole Book was revealed in one go, its remembrance by heart would not have been that easy. Remembrance of the Qur'an by heart expelled all worries from his heart. 2. Whenever an objection was raised by the infidels or a maltreatment was perpetrated, a verse would reveal to give him fortitude. In case the entire Qur’ an was revealed in one piece and the fortitude for the special occasion had also been mentioned in it, its search in the Book would have been painstaking. Moreover, it would have been uncertain whether or not the attention of the Holy Prophet ﷺ would be drawn to the particular verse. 3. Instant response by way of revelation to answer awkward questions was by itself the confirmation of Allah Ta’ ala's support which is the biggest source of strength for the heart. The wisdom of keeping the heart strong is not dependent on gradual revelations alone. Other factors in this regard are mentioned in the following verse of Surah Bani Isra'il وَقُرْاٰنًا فَرَقْنٰهُ لِتَقْرَاَهٗ عَلَي النَّاسِ عَلٰي مُكْثٍ (And We have divided the Qur'an in portion, so that you may recite it to the people gradually - 17:106). (Bayan ul-Qur’ an)