Now to analyze the words of this verse and their positioning:

The fact that "has set a seal (khatama)" is tied to "they will not believe (lâ yu'minûna)" and follows [immediately] after it is just like making the punishment the consequence of the act, as though it is saying, "Since they have corrupted their wills and not come to believe, they have been punished by having their hearts sealed and blocked up."

The word(s) "has set a seal (khatama)" also indicate a compound metaphor, which alludes to parabolic comparisons as a [literary] device (uslûb tamthîlî), which [in turn] signifies a proverb depicting their misguidance, for what it means is the truth being prevented from penetrating to the heart. The term 'seal or sealing' (al-khatm) portrays the heart as a house built by Allâh (May He be exalted!) to be a treasury full of jewels. But due to the misuse of the will, it has become corrupted and has putrified, and what it contains has turned poisonous so it has been locked up and sealed, to make it shunned.

The word "Allâh." know that the use of [the proper noun instead of the first person pronoun] signifies a turning (iltifat) from the first to the third persons. This comprises a subtle point pertaining to the intended (bi'n-niyya) relationship of the word "Allâh" with "they will not believe (lâ yu'minûna)-" I mean that the implied "in Allâh" indicates the following subtle meaning: when the light of the knowledge of Allâh came to them, they did not open the door of their hearts to it, so He turned away from them angrily and locked the door on them.

The use of "on ('alâ)" with the transitive verb "khatama" suggests that the verb includes the meaning of branding, as though it is saying: Allâh has set a seal on their hearts, branding and marking them so the angels see them. "On ('alâ)" also suggests that it is the heart's lofty door that is blocked up, not its lowly door that looks onto this world.

The word "their hearts (qulûbihim)" precedes hearing and sight because it is the seat of belief; and because the first evidences of the Maker are manifested from the heart's consultation with itself, and from the conscience referring to the innate disposition. For when a person consults himself, he feels an acute sense of powerlessness that drives him to seek out a point of support, and he perceives his clamouring need to fulfil his hopes and is compelled to find a source of assistance. But there is no support to be found and no help except in belief.