فَبِمَا نَقْضِهِم مِّيثَـٰقَهُمْ لَعَنَّـٰهُمْ وَجَعَلْنَا قُلُوبَهُمْ قَـٰسِيَةً ۖ يُحَرِّفُونَ ٱلْكَلِمَ عَن مَّوَاضِعِهِۦ ۙ وَنَسُوا۟ حَظًّا مِّمَّا ذُكِّرُوا۟ بِهِۦ ۚ وَلَا تَزَالُ تَطَّلِعُ عَلَىٰ خَآئِنَةٍ مِّنْهُمْ إِلَّا قَلِيلًا مِّنْهُمْ ۖ فَٱعْفُ عَنْهُمْ وَٱصْفَحْ ۚ إِنَّ ٱللَّهَ يُحِبُّ ٱلْمُحْسِنِينَ Qur’an Al-Ma'ida (5:13)Fabima naqdihim meethaqahum laAAannahum wajaAAalna quloobahum qasiyatan yuharrifoona alkalima AAan mawadiAAihi wanasoo haththan mimma thukkiroo bihi wala tazalu tattaliAAu AAala khainatin minhum illa qaleelan minhum faoAAfu AAanhum waisfah inna Allaha yuhibbu almuhsineena
An allusion to their lack of trust in God and their persistent sinning.
See 4:46 , where the same accusation is levelled against the children of Israel.
Cursed them: that means that because of the breach of their Covenant, Allah withdrew His overflowing Grace from them. The withdrawal of Grace made their hearts grow hard in two ways: (1) they were no longer protected from the assaults of evil, and (2) they became impervious even to the message of forgiveness and mercy which is open to all Allah's creatures.
Israel, when it lost Allah's grace as above, began to sin against truth and religion in three ways: (1) they began to misuse Scripture itself, by either taking words out of their right meaning, or applying them to things for which they were never meant; (2) in doing so, they conveniently forgot a part of the Message and purpose of Allah; and (3) they invented new deceits to support the old ones.
Cf. ii. 109 and n. 110, where I have explained the different shades of meaning in the words for "forgiveness."