The first verse is telling us that Bani Isra'il were heedless to clear instructions given to them. Then, they broke the pledge for which they were punished. There were two kinds of punishment which visited Bani Isra'il because of their misdeeds and contumacy:
1.Manifest and perceptible, like the hail of stones and rocks or the overturning of land mass, which find mention in several verses of the Qur'an.
2. Intellectual and spiritual, as if their very minds and hearts Were transformed in punishment for their contumacy which made them incapable of thinking and feeling and they went on sinking deeper in the curse of their sins.
The verse opens with the words: فَبِمَا نَقْضِهِم مِّيثَاقَهُمْ لَعَنَّاهُمْ وَجَعَلْنَا قُلُوبَهُمْ قَاسِيَةً means that it was because of their contravention of the solemn pledge that Allah made them far-removed from His mercy (that is, from its effects, which is the reality of 'La'nah' or curse Hadrat Thanavi) and their hearts hardened against any penetration of truth. This removal from mercy and the hardening of hearts has been likened to " (stain or rust) by the Holy Qur'an in Surah al-Mutaffifin: كَلَّا ۖ بَلْ ۜ رَانَ عَلَىٰ قُلُوبِهِم مَّا كَانُوا يَكْسِبُونَ . It means that the reason why they reject open verses of the Qur'an and all too manifest signs is that the stain of sins has settled down on their hearts. In a hadith, the Holy Prophet (SM) said: men someone commits a sin for the first time, a black dot gets stamped on the heart. He feels the pinch of this evil presence all the time. It is like a black stain on a white cloth which is a constant sore of the eye. Now, if he gets chastened, repents and resolves not to do it again, that dot is erased; and should he become care-free and start a spree of other sins, a black dot for each subsequent sin will keep adding up, so much so, that the clean slate of his heart will turn jet black with these dots of stain. At that stage, the state of his heart will become very much like a pot placed upside down in which nothing goes in but to come out instantly. Therefore, nothing good settles down in his heart for he has fallen into a state of moral inertia to him nothing good is good and no evil is evil. Rather, the opposite becomes his more likely approach he starts taking defect as merit, vice as virtue and sin as reward. Thus armed with his crooked thinking, he goes on riding high in his rebellion and contumacy. This is a cash and spot punishment of his sin which he gets right here in this mortal world.
Some respected elders have said: إن من جزاء الحسنة الحسنة بعدها وان من جزاء السيئة السيئة بعدها, that is, the cash reward of a good deed is that one gets the Taufiq (the ability given by Allah) of doing another good deed. Similarly, the cash reward of a sin is that one's heart starts being attracted by other sins soon after the first. Thus, obedience and disobedience both have a pull of their own one good invites another good and one evil attracts other evils and sins.
Referring back to the breach of trust committed by the Bani Isra'il, it can be said that the cash punishment due against it was that they were removed from Divine mercy which is the greatest medium of salvation. And their hearts became so hardened that they stooped to the level of 'moving words away from their places' which means that they alter the Word of God in words or meanings or recitation all of which have been described in the Holy Qur'an and the books of Ahadith, some of which have been acknowledged by European Christians as well (Tafsir Usmani).
The result of the intellectual and spiritual punishment mentioned above was that they forgot to take advantage of the good counsel given to them: فنسوا حظا مما ذكروا به. Then, this punishment stuck glued to them: ولا تزال تطلع على خائنة منهم that is, the Prophet of Islam would keep knowing about one or the other breaches, deceptions and treacheries committed by them. The exception is indicated in: الا قليلا منهم (but a few) which refers to people like Sayyidna 'Abdullah ibn Salam and others who were followers of the faith of the People of the Book before they became true Muslims.
Upto this point, the description of the misconduct of the Bani Isra'il was obviously demanding enough that the Holy Prophet (SM) should hold them in contempt and never allow them to come to him. Therefore, in the last sentence of this verse, the instruction given to the Holy Prophet (SM) was: فَاعْفُ عَنْهُمْ وَاصْفَحْ ۚ إِنَّ اللَّهَ يُحِبُّ الْمُحْسِنِينَ (So forgive them and forego. Indeed, Allah loves the good-in-deed). It means that, despite their peculiar condition being what it is, it is better that the Prophet of Islam abstains from doing what is otherwise quite natural. In other words, he should not treat them with contempt or hatred because after knowing what they are hardhearted and cold the chances that any good counsel would affect them are very remote. Granted. But, toleration and civility of morals are wonder virtues which could probe out some sense and sensibility from out of such insensate people. Even if this effort to drive some sense into them fails, something worth doing still remains keeping one's own matters and morals correct is, after all, quite necessary. Allah Almighty likes what is done with good grace and it will definitely bring Muslims closer to Him.
From the first verse, 5:13, which dealt with the breach of trust by Jews and their punishment for it, we now move to the next verse, 5:14, which refers to Christians: وَمِنَ الَّذِينَ قَالُوا إِنَّا نَصَارَىٰ. In this verse, the punishment described for Christians is mutual dissension. Being for their breach of trust, this will continue right upto the Last Day.