At-Tawbah 9:125

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 (124)But as for those in whose hearts is a disease, it will add Rijs (doubt) to their Rijs (doubt); and they die while they are disbelievers (125)

Faith of the Believers increases, while Hypocrites increase in Doubts and Suspicion

Allah said,

وَإِذَا مَا أُنزِلَتْ سُورَةٌ

(And whenever there comes down a Surah), then among the hypocrites are,

مَّن يَقُولُ أَيُّكُمْ زَادَتْهُ هَٰذِهِ إِيمَانًا

(some who say: "Which of you has had his faith increased by it?")

They say to each other, who among you had his faith increased by this Surah [from the Qur'an]? Allah the Exalted said,

فَأَمَّا الَّذِينَ آمَنُوا فَزَادَتْهُمْ إِيمَانًا وَهُمْ يَسْتَبْشِرُونَ

(As for those who believe, it has increased their faith, and they rejoice.)

This Ayah is one of the mightiest evidences that faith increases and decreases, as is the belief of most of the Salaf and later generations of scholars and Imams. Many scholars said that there is a consensus on this ruling. We explained this subject in detail in the beginning of the explanation of Sahih Al-Bukhari, may Allah grant him His mercy.

Allah said next,

وَأَمَّا الَّذِينَ فِي قُلُوبِهِم مَّرَضٌ فَزَادَتْهُمْ رِجْسًا إِلَىٰ رِجْسِهِمْ

(But as for those in whose hearts is a disease, it will add Rijs to their Rijs.) the Surah increases them in doubt, and brings more suspicion on top of the doubts and suspicion that they had before. Allah said in another Ayah,

وَنُنَزِّلُ مِنَ الْقُرْآنِ مَا هُوَ شِفَاءٌ

(And We send down in the Qur'an that which is a healing)[17:82], and,

قُلْ هُوَ لِلَّذِينَ آمَنُوا هُدًى وَشِفَاءٌ ۖ وَالَّذِينَ لَا يُؤْمِنُونَ فِي آذَانِهِمْ وَقْرٌ وَهُوَ عَلَيْهِمْ عَمًى ۚ أُولَٰئِكَ يُنَادَوْنَ مِن مَّكَانٍ بَعِيدٍ

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

This indicates the misery of the hypocrites and disbelievers, since, what should bring guidance to their hearts is instead a cause of misguidance and destruction for them. Similarly, those who get upset by a type of food, for instance, will be upset and anxious even more if they are fed that food!