The increase of Iman is explicitly cited in many verses of the Quran. For example, Allah says: "For believers are those who, when Allah is mentioned, feel a tremor in their hearts, and when they hear His Signs rehearsed, find their belief strengthened..." (8:2). Thus, if one feels a tremor in his heart when he hears the Signs of Allah rehearsed [not only when the Signs are revealed], his Iman increases. Indeed, a believer experiences this. When he hears the Signs of Allah rehearsed, his understanding of the meanings of the Quran becomes much clearer, his tendency to do good and his fear of committing evil is strengthened, and his knowledge of Allah and his desire to obey Him becomes stronger. Hence, his Iman increases. Similarly, Allah says: "Those [believers] to whom the people [hypocrites] said: 'A great army is gathering against you: and frightened them: but it [only] increased their belief:' They said: 'For us Allah suffices, and He is the best Disposer of affairs"' (3:173). This increase in belief was due to their saying:

"For us Allah suffices, and He is the best Disposer of affairs," when they were frightened by their enemy [not when the verse was revealed]. Allah also says:

"Whenever there comes down a Surah [a chapter from the Quran], some of them say: 'Which of you has had his belief increased by it?' Yes, those who believe, their belief is increased, and they do rejoice. But those in whose hearts is a disease, it will add doubt to their doubt..." (9:1225). This increase is not attributed to their affirmation that this Surah was revealed by Allah, but is due to their better understanding and practice of the meanings of the Quran. Thus, if it is a command to resort to jihad, their desire [to do this] becomes stronger, and if it is a prohibition from doing an evil, their hatred for doing it becomes stronger. For this reason, Allah says: "And they do rejoice." Here, the word "rejoice" is not mere affirmation. Allah also says: "Those to whom We have given the Book rejoice at what has been revealed unto you: but there are among the clans those who reject a part thereof..." (13:36). Here, rejoicing is a response to the increase of Iman. Allah says: "Say in the Bounty of Allah. And in His Mercy, in that let them rejoice. . ." (10:58); and ". . . On that Day shall the believers rejoice. With the help of Allah…." (30:5). Allah also says: "And We have set none but angels as guardians of the Fire; and We have fixed their number only as a trial for unbelievers, in order that the people of the Book may arrive at certainty, and the believers may increase in belief…." (74:31); and "It is He who sent down tranquility into the hearts of the believers, that they may add belief to their belief." (48:4).

This verse was revealed when the Prophet (sallallaahu alayhi wasallam) came back with his Companions from the Treaty of Hudaybiyyah. Allah makes tranquility a condition for the increase of Iman. The word "tranquility" refers to peace of mind but not to the knowledge and certainty of the heart. Supporting this is His saying on the day of Hunayn. Allah says: "But Allah did pour His calm on his Messenger and on the believers, and sent down forces [angels] which you saw not..." (9:26); and "They two were in the Cave, and he said to his companion: 'Have no fear, for Allah is with us.' Then Allah sent down His peace upon him, and strengthened him with forces [angels] which you saw not..." (9:40). Indeed, on the day of Hunayn and on the day of the Cave, none of the verses of the Qur an were revealed. But, Allah sends down His tranquility and peace into the hearts of the believers. Therefore, they came back from Hudaybiyyah in order to increase their Iman.