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:
[قُلْ هُوَ 
لِلَّذِينَ ءَامَنُواْ هُدًى وَشِفَآءٌ وَالَّذِينَ لاَ يُؤْمِنُونَ فِى 
ءَاذَانِهِمْ وَقْرٌ وَهُوَ عَلَيْهِمْ عَمًى أُوْلَـئِكَ يُنَادَوْنَ مِن مَّكَانٍ 
بَعِيدٍ]
(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]
[وَإِذَا مَآ 
أُنزِلَتْ سُورَةٌ فَمِنْهُمْ مَّن يَقُولُ أَيُّكُمْ زَادَتْهُ هَـذِهِ إِيمَـناً 
فَأَمَّا الَّذِينَ ءامَنُواْ فَزَادَتْهُمْ إِيمَـناً وَهُمْ يَسْتَبْشِرُونَ - 
وَأَمَّا الَّذِينَ فِى قُلُوبِهِم مَّرَضٌ فَزَادَتْهُمْ رِجْسًا إِلَى رِجْسِهِمْ 
وَمَاتُواْ وَهُمْ كَـفِرُونَ ]
(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.
[وَنُنَزِّلُ مِنَ 
الْقُرْءَانِ مَا هُوَ شِفَآءٌ وَرَحْمَةٌ لِّلْمُؤْمِنِينَ]
(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.''
[وَلاَ يَزِيدُ 
الظَّـلِمِينَ إَلاَّ خَسَارًا]
(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.
[وَإِذَآ أَنْعَمْنَا 
عَلَى الإنْسَـنِ أَعْرَضَ وَنَأَى بِجَانِبِهِ وَإِذَا مَسَّهُ الشَّرُّ كَانَ 
يَئُوساً ]
[قُلْ كُلٌّ يَعْمَلُ 
عَلَى شَاكِلَتِهِ فَرَبُّكُمْ أَعْلَمُ بِمَنْ هُوَ أَهْدَى سَبِيلاً ]
(83. And when We bestow Our grace on man, he turns away and becomes arrogant. 
And when evil touches him, he is in great despair.) (84. Say: "Each one does 
according to Shakilatihi, and your Lord knows best of him whose path is 
right.'')