The Quran has been revealed in two ways. The first was in the form of the revelation of entire chapters all at once (daf’i/دفعی), and the other was gradual revelations of verses. Several reasons have been mentioned for the gradual revelation of the Quran, of which are mentioned below:
1. In order to render the heart of the Prophet (pbuh) firm and unwavering.
2. The continuous and gradual revelation of verses strengthened the hearts and beliefs of the Prophet (pbuh) and Muslims.
3. A gradual revelation of Quran would allow society to reflect and ponder upon that which was revealed. It would also provide them which a sufficient opportunity to memorize the verses, and put that which they had learned into practice.
Considering the fact that the Holy Quran encompasses a vast number of topics, it was necessary to separate and categorize the verses into groups which conveyed a common message. For this reason, the Quran is comprised of different chapters and verses.
It should be mentioned that the structure and placement of each verse was in accordance to the specific orders of the Holy Prophet (pbuh), therefore there is no doubt in regards to the legitimacy of the Quran’s composition.
However, certain chapters were revealed in whole at one time, as opposed to a gradual revelation of the verses. This applies to some of the shorter chapters in the Quran whose names will be mentioned in the detailed answer.
Without a doubt, it is known that the Holy Quran was revealed to the Holy Prophet (pbuh) gradually over the course of twenty three years. On the other hand, we have read that “The Quran was revealed in the month of Ramadan and (its revelation occurred) in one night”. The mentioned verse attests to the fact that the Quran was revealed in the month of Ramadan. In addition to this, the verses “إِنَّا أَنْزَلْناهُ فِی لَیْلَةِ الْقَدْرِ” (Indeed We sent it down on the Night of Ordainment) and “إِنَّا أَنْزَلْناهُ فِی لَیْلَةٍ مُبارَکَةٍ” (Indeed We have sent it down on a blessed night) state that the Quran was revealed in one night. Upon comparing these three verses, it becomes clear that the Quran was revealed in one night in the month of Ramadan.
However, narrations substantiate the claim that the Quran was revealed in two different manners:
Hafs ibn Ghiyath narrates: I asked Imam Sadiq (as), how is it that God states “شَهْرُ رَمَضانَ الَّذِی أُنْزِلَ فِیهِ الْقُرْآنُ” (The month of Ramadan is one in which the Quran was sent down) when in fact the duration of its revelation from beginning to end took twenty years? The Imam stated: The entire Quran was revealed in the month of Ramadan to Baytul Ma’mur, after which it was revealed over the course of twenty years. In the narrations of the Sunni school of thought, Baytul Izzah has been mentioned in place of Baytul Ma’mur. In the tafsir (commentary of the Quran) of Safi, it has been mentioned that Baytul Ma’mur is in reference to the heart of the Messenger of God (pbuh). It states: It seems what is meant [by Baytul Ma’mur] is the heart of the Prophet (pbuh), for Allah the Almighty states: “نَزَلَ بِهِ الرُّوحُ الْأَمِینُ عَلى قَلْبِکَ” (brought down by the Trustworthy Spirit upon your heart). In the twenty years that followed, the Prophet gradually brought that which was in his heart to speech when the angel Jibrayil descended upon him with the revelation and its words.
The reasons for which the Quran was gradually revealed:
During the life of the Prophet (pbuh), chapters and verses of the Quran were often revealed in cases such as: when an incident had occurred, in response to a question that was asked, or in cases when Muslims faced a problem or difficulty. Not only would the verse then be revealed, but the context and circumstances surrounding its revelation would prove vital in understanding the exact meaning of many verses within the Quran.
Therefore, the Quran was gradually revealed in the span of twenty three years; meaning its revelation took place before and after the hijrah of the Prophet (pbuh) and during a myriad of different incidents and events that occurred. These revelations, which sometimes came in the forms of individual verses and at other times as entire chapters, continued until the last year of the life of the Holy Prophet (pbuh). At that point, all of the verses were compiled into a book titled the “Quran”.
This manner of revelation is one of the characteristics which make the Quran unique from other divinely revealed books. The scrolls of Ibrahim and tablets of Musa (as) were revealed in whole at one time, and for this reason, the mushrikin (polytheists) used this as a pretext for finding fault in the Quran. The Quran states: “The faithless say, ‘Why has not the Quran been sent down to him all at once?’”. In response, it is said: “So it is, that We may strengthen your heart with it, and We have recited it [to you] in a measured tone.” In another verse it is mentioned: “We have sent the Quran in [discrete] parts so that you may read it to the people a little at a time, and We have sent it down piecemeal.” Allamah Tabatabai (may Allah’s mercy be upon him), has provided valuable insight and commentary into this verse in regards to which he states: “…this verse covers all Quranic teachings that are with Allah (swt) and is saying that these teachings were sent down in the form of words, thus making them only receivable gradually [this is one of the traits of worldly matters; time affects them and makes them gradual), this was done so that it would be easier for the people to understand and learn these teachings. This is the message the verse (جَعَلْناهُ قُرْآناً عَرَبِیًّا لَعَلَّکُمْ تَعْقِلُونَ وَ إِنَّهُ فی أُمِّ الْکِتابِ لَدَیْنا لَعَلِیٌّ حَکیم) is also conveying and trying to explain. The reason for why Quranic verses were revealed verse by verse and surah by surah is because of the development of the people’s potential to receive the fundamental teachings and practical rulings and also because of certain benefits this had for the believers, such as putting into practice each verse that was revealed rather than them merely memorizing it and them not becoming tired of its teachings (because when too many teachings are revealed at once, it is hard to learn of all of them altogether) as was the case with the Torah; the Jews became tired of it and neglected practicing it as a result of it being sent down all at once, until Allah (swt) was forced to hold a mountain over their heads in order for them to adhere to it”.
Another reason for the Quran’s gradual revelation may have been so that the Prophet (pbuh) and the Muslims would feel that the mercy of their Lord had encompassed them in a unique manner, which in turn would strengthen the relationship between themselves and God. The continuity of its revelation would enlighten and strengthen the hearts of the believers and reinforce their faith.
After the revelation of these verses in various times and occasions, they were sorted into chapters and compiled into a book by the name of “The Quran”. The number of verses in each chapter was an order directly specified by the Holy Prophet (pbuh). Meaning, whether the chapter was small in size (such as Surah al-Kawthar which consists of three verses) or large (such as Surah al-Baqarah which consists of 286 verses), its order and size was in accordance to what the Prophet (pbuh) had stated. To this day, the makeup of the Quran has not been altered or modified in the least, which itself is one of the miracles of this divine book.
The following should be mentioned in regards to the method of compilation of chapters from various verses of the Quran: As previously mentioned, the number of verses found in each chapter and their assortment was an order given by the Prophet (pbuh) during his lifetime and is something that must be accepted without question and observed during recitation. Therefore, there is no doubt in regards to the legitimacy of the Quran’s composition. Upon revelation, each chapter began with “بسم الله الرحمن الرحیم” (In the Name of Allah, the All-beneficent, the All-merciful), and from then on, each verse would be written in the order of its revelation. This would continue until a new “بسم الله” (In the Name of Allah) was revealed, thus signifying the beginning of a new chapter. This manner served as the natural order of the Quran’s verses. However, on occasion the angel Jibrayil would tell the Prophet (pbuh) to place the verses in an assortment contrary to the order in which they were revealed, therefore the he would order the verse to be placed in another specified chapter. An example of this can be seen in regards to the following verse: “وَ اتَّقُوا یَوْماً تُرْجَعُونَ فیهِ إِلَى اللَّهِ ثُمَّ تُوَفَّى کُلُّ نَفْسٍ ما کَسَبَتْ وَ هُمْ لا یُظْلَمُونَ” (And beware of a day in which you will be brought back to Allah. Then every soul shall be recompensed fully for what it has earned, and they will not be wronged). This particular verse was amongst the final verses revealed to the Prophet (pbuh), however, he ordered that it be placed as verse 281 in Surah al-Baqarah, between the verses concerning riba (prohibited interest) and religion. Therefore, the order of the Quran’s verses, whether they appear in their usual arrangement or otherwise, was under the supervision and guidance of the Holy Prophet (pbuh) himself. It should be noted that a verse of the Quran does not necessarily end simultaneously with the topic being discussed in that verse. Many a time, the verse will end and the topic being discussed therein will be continued in the next verse. Therefore, the length of a verse is purely divinely ordained, and is irrelevant to the verse’s content and topic. In regards to the size of the verses, there is a minor difference of opinion. During the time of the Holy Prophet (pbuh), when he would recite the verses to the people, he would sometimes pause in the middle of his recitation, while not pausing in other recitations of the same verse. As a result, people would assume that the Prophet (pbuh) had completed reciting the verse. This caused a minor difference of opinion in regards to the length of a small number of verses.
The following chapters of the Quran have been revealed to the Holy Prophet (pbuh) in entirety at one time:
al-Duha (Morning Brightness), al-Fatihah (The Opening), al-Ikhlas (Monotheism), al-Kawthar (Abundance),
al-Masad (Palm Fiber), al-Bayyinah (The Proof), al-Nasr (Help), al-Nas (Humans), al-Falaq (Daybreak),
al-Mursalat (The Emissaries), al-Ma’idah (The Table), al-Anʻam (Cattle), al-Tawbah (Repentance), al-Saff (Ranks), al-ʻAdiyat (The Chargers), al-Kafirun (The Faithless).
 Baqarah:185 “شهر رمضان الذی انزل فیه القرآن”
 Kafi, quoted by tafsir Safi, ninth introduction.
 Furqan: 32 “وَ قالَ الَّذینَ کَفَرُوا لَوْ لا نُزِّلَ عَلَیْهِ الْقُرْآنُ جُمْلَةً واحِدَةً کَذلِکَ لِنُثَبِّتَ بِهِ فُؤادَکَ وَ
 Isra:106 “وَ قُرْآناً فَرَقْناهُ لِتَقْرَأَهُ عَلَى النَّاسِ عَلى مُکْثٍ وَ نَزَّلْناهُ تَنْزیلا”
 Zukhruf:3 and 4.
 The growth and development of the people’s potential.
 Muhammad Husein Tabatabai, translation of Al-Mizan, vol. 13, pp. 305 d 306.
 Muhammad Hadi Ma’refat, Ulume Qurani, pp. 60-61.
 Ulume Qurani, pg. 111.
 Ulume Qurani, pg. 117.
 Mostafa Asrar, Danestanihaye Quran, pg. 28; Sadeq Hasanzadeh, Kilide Quran, pg. 134.