It says: "So, for their breaking their covenant .
It says: "So, for their breaking their covenant ..." Following to this breach of promise, they rejected the Signs of Allah and paved the way of opposition. ".....and their disbelief in the Signs of Allah ..." They did not suffice only to this wrong-doing, but also they set their hands to another great crime. They killed the leaders and the guides of the way of Truth, viz. the prophets. They slew the prophets without having any reason for their action.
".....and their slaying the prophets wrongfully ..." They were so insolent in committing offences that they mocked the statements of the prophets and they used to explicitly say to them that their hearts were locked so that they hindered them to hear and accept the invitation of the prophets.
".....and their saying 'Our hearts are covered'; ..." Here, the Qur'an adds a sense that their hearts are sealed totally and no truth penetrates into them, but the reason of that situation is their own infidelity and disbelief. That is why they do not believe in truth, except a small group of them who avoid these kinds of obstinacies and that they pave the path of truth. It says: "... nay! but Allah sealed them for their disbelief, So they do not believe save a few.
Therefore, calumny is counted in the same row with infidelity, and, consequently, a grievous calumny deserves a grievous chastisement. "And because of their disbelief and their speaking against Mary a grievous calumny" However, in an immoral society, there may sometimes happen that the most disgraceful calumnies be attributed to the most immaculate persons.