Surah 17. Al-Israa

Allah tells us that His Book, which He has revealed to His Messenger Muhammad , the Qur'an to which falsehood cannot come, from before it or behind it, (it is) sent down by the All-Wise, Worthy of all praise, is a cure and a mercy for the believers, meaning that it takes away whatever is in their hearts of doubt, hypocrisy, Shirk, confusion and inclination towards falsehood. The Qur'an cures all of that. It is also a mercy through which one attains faith and wisdom and seeks goodness. This is only for those who believe in it and accept it as truthful, it is a cure and a mercy only for such people. As for the disbeliever who is wronging himself by his disbelief, when he hears the Qur'an, it only makes him further from the truth and increases him in his disbelief. The problem lies with the disbeliever himself, not with the Qur'an, as Allah says:

(Say: "It is for those who believe, a guide and a cure. And as for those who disbelieve, there is heaviness in their ears, and it is blindness for them. They are those who are called from a place far away (so they neither listen nor understand).'') ﴿41:44﴾

(And whenever there comes down a Surah, some of them (hypocrites) say: "Which of you has had his faith increased by it'' As for those who believe, it has increased their faith, and they rejoice. But as for those in whose hearts is a disease, it will add suspicion and doubt to their suspicion, disbelief and doubt; and they die while they are disbelievers.) (9:124-125) And there are many other similar Ayat.

(And We send down of the Qur'an that which is a cure and a mercy to the believers,) Qatadah said, "When the believer hears it, he benefits from it and memorizes it and understands it.''

(and it increases the wrongdoers in nothing but loss.) They do not benefit from it or memorize it or understand it, for Allah has made this Qur'an a cure and a mercy for the believers.