In their hearts is a sickness, in other words, doubt and hypocrisy. The use of the indefinite form of mara? (sickness) and the adverbial clause is an indication of the fact that [such a] sickness has befallen them and has established itself and taken root in these [hearts of theirs, as we have mentioned in the above structure, for otherwise He would have said: 'Their hearts are sick or [that they are] dead'; and God has increased them in sickness, that is, [with] another [sickness]: [God increases their] spite, envy and rancour by exalting the word of religion and the granting of assistance to the Messenger and the believers. All despicable acts are sicknesses of the heart as they are cause of its weakening and the evil of its private actions and its ruin in the final issue. He distinguishes between the two chastisements by [stipulating] pain for the hypocrites and an awesome [chastisement] for disbelievers, because the chastisement for those who have been banished from pre-eternity is greater. Thus they do not experience the intensity of its painfulness because of the impurity of their hearts' perception, similar to the state of a dead or hemiplegic or numbed limb [that senses nothing] when it undergoes severing or cauterisation and other kinds of pain. As for the hypocrites, they sense the extreme pain because of their established preparedness [for belief] at the outset and the permanence of their [faculties of] perception. Inevitably then their chastisement is painful, caused as it is by the chronic sickness that occurs [in their hearts], namely, mendacity and its [evil] concomitants.
and there awaits them a painful chastisement because they used to lie.