﴿ 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 -”

And those who disbelieve say ‘Why has the Qur’ān not been revealed to him all at once?’ as the Torah the Gospel and the Book of Psalms were. God exalted be He says We have revealed it thus in parts it is that We may strengthen your inner-heart with it and We have arranged it in a specific order that is to say We produce it in stages one part after another gradually and deliberately in order to facilitate its comprehension and its memorisation.