Allāh says:

خَتَمَ اللَّهُ عَلَىٰ قُلُوبِهِمْ وَعَلَىٰ سَمْعِهِمْ ۖ وَعَلَىٰ أَبْصَارِهِمْ غِشَاوَةٌ ۖ وَلَهُمْ عَذَابٌ عَظِيمٌ

‘Allāh has placed a seal upon their hearts and upon their hearing, and over their vision is a veil (because they showed no desire for truth, goodness and guidance after it had been made clear to them). And for them is a great punishment (for their stubborn disbelief).’
[Al-Baqarah, 2:7]

Imām Muḥammad ibn ʿAlī al-Shawkānī [d. 1250 AH] explains:

Narrated by Ibn Jurayj who said: A seal is placed on their hearts and their hearing, and a veil is placed on their vision as Allāh says:

فَإِن يَشَإِ اللَّهُ يَخْتِمْ عَلَىٰ قَلْبِكَ

‘But if Allah willed, He could seal over your heart.’
[Al-Shūrā, 42: 24]

Additionally He says:

وَخَتَمَ عَلَىٰ سَمْعِهِ وَقَلْبِهِ وَجَعَلَ عَلَىٰ بَصَرِهِ غِشَاوَةً

‘And has set a seal upon his hearing and his heart and put over his vision a veil?‘
[Al-Jāthiyah, 45: 23]

Ibn Jarīr (al-Ṭabarī) said concerning the meaning of this ‘seal’: The truth with regards to it is only what has been similarly confirmed from the Messenger of Allāh (صلى الله عليه وسلم), then he mentions the chain of narrators to Abū Hurayrah who said: the Messenger of Allāh (صلى الله عليه وسلم) said:

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Verily, when a believer commits one misdeed, a black spot is placed on his heart. If he is to repent, refrain [from the same sin], and work tirelessly [in its avoidance], his heart will regain its purity. If he increases [in sin], the [black marks] shall increase until they have completely overcome the heart.

Such is the stain that Allāh has said concerning:

كَلَّا ۖ بَلْ ۜ رَانَ عَلَىٰ قُلُوبِهِم مَّا كَانُوا يَكْسِبُونَ

‘No (the affair is not as they presume, it is Allāh’s Speech and Revelation to His Prophet)! Rather, what they used to earn (of evil deeds) has covered their hearts with a stain (which prevents them from recognising and desiring the truth).’
[Al-Muṭaffifīn, 83:14]

This [ḥadīth] with the same narration has also been mentioned by al-Tirmidhī and graded authentic by al-Nasāʾī. Then Ibn Jarīr [al-Ṭabarī] said: The Messenger of Allāh (صلى الله عليه وسلم) has related to us that if sins are committed continuously, they result in the closing of the heart [of the sinner]. If the heart is closed, a seal and wrap is brought from Allāh the Glorified such that there remains no route of entry to it [for guidance] and from it there will exist no exit route for disbelief. Such is the seal that Allāh has mentioned in His saying: ‘Allāh has placed a seal upon their hearts and upon their hearing.’ It is similar to the wrap or covering that shields the vision, sealing it encompassingly, preventing its comprehension of that which it gazes upon until it is slit, and removed in its entirety. Thus, true belief shall never enter the hearts of those described by Allāh as having sealed hearts except after that seal is broken, and its attachment to it is forgone.

Source: Fatḥ al-Qadīr 1: 42
Translated by: Riyāḍ al-Kanadī