Taberî explains this verse by stating that those with sickness in their hearts are hypocrites, and the word 'marad' (sickness) here is used not in the sense of physical illness but in the sense of doubt in religion and hypocrisy. The mention of heart sickness is actually an indirect expression through metonymy of their doubt and hesitation in faith; they neither believe with confirmation nor deny with disbelief, but rather waver between the two.

وإنما عنى تبارك وتعالى بخبره عن مرض قلوبهم الخبر عن مرض ما في قلوبهم من الاعتقاد، ولكن لما كان معلوما بالخبر عن مرض القلب أنه معني به مرض ما هم معتقدوه من الاعتقاد استغنى بالخبر عن القلب بذلك، والكناية عن تصريح الخبر عن ضمائرهم واعتقاداتهم

Zemahşerî in this passage connects the establishment of hypocrisy in the hearts of hypocrites and their inability to be freed from it until death to their failure to keep their promise of charity and prayer to Allah and their being exposed as liars. He also specifies that the pronoun refers to Allah, explaining that Allah left them to ruin/hypocrisy ('hazele') for this reason. He also makes reference to a statement regarding the fact that turning back from one's promise is considered a sign of hypocrisy.

والمعنى : فخذلهم حتى نافقوا وتمكن في قلوبهم نفاقهم فلا ينفك عنها إلى أن يموتوا بسبب إخلافهم ما وعدوا الله من التصدق والصلاح وكونهم كاذبين . ومنه : جعل خلف الوعد ثلث النفاق

Râzî begins by defining heart sickness ('maraz'): sickness is a quality that causes harm in the actions that proceed from the place where that quality exists; since the inherent effect specific to the heart is knowledge of Allah, obedience to Him, and worship of Him, when qualities form in the heart that obstruct these effects, these are counted as heart sickness. He conveys five reasons of the Mu'tazila regarding the fact that it is not permissible for Allah to create disbelief and ignorance as actions, and argues that for this reason the expression 'fa zâdahümullâhu maradan' (so Allah increased them in sickness) needs to be reinterpreted. As an interpretation, he states that sickness means 'grief/sorrow' and that when the hypocrites saw that the Prophet's work was rising day by day, their sorrow increased, and Allah added sorrow to this sorrow; secondly, he notes that with the increase in religious obligations, their disbelief and sicknesses increased.

فإن قيل : الزيادة من جنس المزيد عليه ، فلو كان المراد من المرض ههنا الكفر والجهل لكان قوله : محمولا على الكفر والجهل ، فيلزم أن يكون الله تعالى فاعلا للكفر والجهل . قالت المعتزلة : لا يجوز أن يكون مراد الله تعالى منه فعل الكفر والجهل لوجوه

According to Kurtubî, the sickness in the expression 'there is sickness in their hearts' is a metaphor expressing the corruption in the beliefs of hypocrites, and this is either doubt and hypocrisy or denial and falsification. For the expression 'Allah increased their sickness,' two views are reported: one being that this is a curse against the hypocrites, and the other being that Allah is reporting that their sickness increased. The people of meaning ('arbâb-ı meânî) interpreted this as inclination toward the world, love for it, and heedlessness of the afterlife, and Junayd said that heart sicknesses arise from following the self, while bodily sicknesses arise from the organs.

والمرض عبارة مستعارة للفساد الذي في عقائدهم وذلك إما أن يكون شكا ونفاقا وإما جحدا وتكذيبا والمعنى : قلوبهم مرضى لخلوها عن العصمة والتوفيق والرعاية والتأييد

According to this exegete, the expression 'sickness in their hearts' primarily means doubt ('shak'); Ibn Abbas, Mujahid, Ikrimah, Hasan al-Basri, Abu'l-Aliyah, Rabî' b. Enes, and Katadah hold this view. In another narration that Dahhâk transmitted from Ibn Abbas, however, this sickness is explained as hypocrisy, and the sickness that Allah increased is interpreted as the increase of their hypocrisy. Abdurrahman b. Zayd b. Aslem noted that this is not a bodily but a religious sickness—the doubt that fell into the hypocrites when they entered Islam.

في قلوبهم مرض قال شك وكذلك قال مجاهد وعكرمة والحسن البصري وأبو العالية والربيع أبن أنس وقتادة وعن عكرمة وطاوس يعني الرياء وقال الضحاك عن ابن عباس في قلوبهم مرض قال نفاق فزادهم الله مرضا قال نفاقا

According to Beydâwî, 'maraz' is used both in its real sense of bodily sickness and metaphorically for spiritual defects (ignorance, bad belief, envy, rancor, love of sin); the verse can carry both meanings. The hearts of the hypocrites suffered pain from the regret and envy they felt upon seeing the elevation of the Prophet's work, and Allah increased this pain; moreover, the disbelief and bad belief in their hearts also increased by their nature. According to another possibility, 'maraz' is the cowardice and weakness that befell their hearts when they saw the strength of the Muslims and their support by the angels.

والآية الكريمة تحتملهما فإن قلوبهم كانت متألمة تحرقا على ما فات عنهم من الرياسة، وحسدا على ما يرون من ثبات أمر الرسول صلى الله عليه وسلم واستعلاء شأنه يوما فيوما، وزاد الله غمهم بما زاد في إعلاء أمره وإشادة ذكره