The disbelief of the Children of Israel, their killing of some of their prophets and their slander of the Virgin Mary 4:155-156
4:155 So, for their (the Children of Israel) breaking of the covenant, and disbelieving the verses of Allah, and killing their Prophets without right and for saying, 'Our hearts are covered' no, but Allah has sealed them for their disbelief, so, except for a few, they do not believe.