At the outset, we will consider two Qur`anic verses, thereafter elucidating the answer to the above question.

In the first verse, we read:

 “As for the faithless, it is the same to them whether you warn them or do not warn them, they will not have faith. Allah has set a seal on their hearts and their hearing, and there is a blindfold on their sight and there is a great punishment for them.”[1] 

In the second verse we read:

“Allah has set a seal on their hearts, so they do not know.”[2]

The meanings of Khatm, Tab’, and Qalb

Khatm is contrasted to fath (to “begin” or “start”) and means “to complete something” or “to reach the end”. The reason why khatm is translated as “sealing” is that the seal in a letter indicates its end. A letter is sealed when it is finished, barring the addition of new material.[3]

Tab‘ also denotes the act of sealing (in which sense it is close in meaning to khatm) as well as sketching, imprinting, and forging metals into coins.[4]

Qalb is employed in the Qur`an in different senses, such as soul, heart, self, intellect, knowledge, etc.[5] 

Nevertheless, it can generally be said that the human being possesses two types of heart: the corporeal heat and the spiritual heart. The corporeal heart is, in physiological terminology and in the vernacular, a muscular organ with the peculiar shape, whose function is the circulation and purification of blood, and in most human beings rests in the left section of the chest. The spiritual heart is one and the same with the spirit and the psyche of the human being. However in the ethical and gnostic terminologies and also in the vocabulary of the Qur`an and the hadiths, heart is used in the latter sense, and as such is the conduit for the conveyance of Divine inspirations and Revelation and the means of achieving Divine knowledge and witnessing the epiphanies of the Truth. It is the locus of human emotions and the root of the exalted human intentions and aspirations.

The point in common between the two usages of qalb (heart) has been expounded as follows. Qalb literally means change and transformation. The muscular heart derives its name from the responsibility to change the blood and purify it. Similarly, as human emotions and intentions are in constant flux and change, their locus has been termed qalb. 

Allah’s (awj) seal on the hearts

In the Qur`an, various terms have been used to refer to the incorrigibility of the hearts of the unbelievers, hypocrites, and the spiteful; among them khatm (sealed), tab‘ (sealed), sarf (turned away), qufl (locked), marad (ill), rayn (tainted), etc. Of course, the shutting of the heart and its corruption is not exclusive to the unbelievers and hypocrites. The heart of the human being—including the believer—is twisted and sealed and deprived of comprehending the Divine Signs in proportion as it is contaminated with the dross of sin.

Thus, sealing the heart refers to the shutting of this conduit for receiving Divine knowledge and inspiration, and the means of this sealing can be inferred from the hadiths mentioned below.

Zurarah has narrated that Imam Muhammad b. ‘Ali al-Baqir (ع) as having said: “There is not a servant but that his heart contains a white slate. When he commits a sin, a black dot appears on it. Thereafter, if he repents, that black dot will disappear, but if he continues the sin, the blackness will expand, such that it will ultimately cover the entire whiteness. After the white slate has been entirely covered, the possessor of that heart will never return to good and virtue. And this is the meaning of the word of God where He says,  

‘No indeed!  Rather their hearts have been sullied by what they have been earning.’”[6] ^and [7]

There are certain factors that cause the accumulation of dross in the heart. In the Noble Qur`an the following issues are enumerated as accounting for the sealing of the heart: disbelief[8]; heedlessness and continuous indifference[9]; vow-breaking and brazen sinfulness[10]; obstinacy and distorting the Divine Word[11]; self-willedness and acting against one’s knowledge[12]: causing corruption, dishonouring family ties[13]. Thus, the veils that cover one’s heart and the obstacles that impede the function of the spiritual heart, hearing, and sight are the results of one’s own volitional conduct. The Divine seal is set by way of punishment and not gratuitously. For, there exists a necessary and definite link between one’s conduct, intentions, and thoughts and their effects, which is inescapable. The reason that this scheme is attributed to Allah (awj) is that the causal relationship between human conduct, intentions, and thoughts and their effects is a Divinely destined and decreed process which is immutable, except in cases where other factors—such as repentance, Divine alerts, tribulations, or the help of a Possessor of the Breath, i.e., a friend of Allah (awj)—affect the human being, bringing about his spiritual transformation, purging his heart of the dross of sin, and once again rejuvenating him with the capacity to be guided.  

In other words, Divine destiny and decree are but a part and parcel of the regulations governing the cosmos and the necessity of an effect following its complete cause. And in the case of the volitional acts of the human being, will, decision, and intention are among the essential components of the complete cause and hence are necessary to bring about his volitional acts. When the human being performs a certain action, its effects inescapably leave their impression on the human soul and psyche. In this light, if the human being himself prepares the grounds for Allah (awj) sealing his heart, by the Law of the cosmos, its effects will haunt him.

In the light of what has been said the following points can be highlighted:

1.    Allah (awj) sealing of the heart is the direct outcome of the volitional conduct of the human being.

2.    As the sealing of the heart is part of the cosmic scheme of Divine destiny and decree, it is attributed to Allah (awj).

3.    The human being whose heart has been tainted by the dross of sin can add to his spiritual contamination by repeating the past mistakes. It should be noted that this perpetuation of sin is on his own accord. 

4.    Although it is very difficult for the sinful human being whose heart is covered with the dross of sin to return to the path of righteousness and is very unlikely, it is not impossible. He can choose with a firm resolution to embark on purging his heart of the darkness of sin. In other words, if the soul and heart of the unbeliever and hypocrite become sealed and dark and debased such that no bright space remains, he is, to employ the words of the Qur`an, finished and there is no hope of his redemption. He has intentionally removed himself from the path of guidance and light and thus has shut the door of repentance on himself.