What does it mean by the Qur’anic expression, in whose hearts is a disease? The Qur’an clearly shows that hearts do become diseased. Therefore, some hearts may also be healthy.

According to the Qur’an, when hearts become diseased, they go out of straight nature and deviate from the right path. In short, in means the involvement of the heart with some sort of suspicion and doubt that pollutes the faith in God and removes confidence in His signs. In other words, the heart blends with polytheism.

On the flip side, a healthy heart means it is steadfast and it adheres to the right path which leads a man to sincerity in belief.

However, there is an interesting contrast between the hypocrites and those in whose hearts is a disease. God says:

• And when the hypocrites were saying, as well as those in whose hearts is a disease, God and His Messenger promised us naught but delusion. (33:12)

The author of al-Mizan says:

It is evident from the above Qur’anic expression, those in whose hearts is a disease, that it is other then the hypocrites… It is because the hypocrites are those who believed with their tongues and their hearts did not believe, while total disbelief is the death of the heart, not a disease. Thus (the expression), those in whose hearts is a disease, are people of weak faith.

– Allamah Tabataba’i (English Vol. 10)

The Qur’an has explained that the heart’s disease, like that of the body, sometimes goes on increasing until it becomes cancerous and end in death of the heart.