Surah 2. Al-Baqara

9That is, they refused to believe and fulfil all or any of the above mentioned six conditions.

10It does not mean that Allah had sealed up their hearts, so they refused to believe. But it means that when they rejected the above-mentioned pre-requisites of belief and chose and adopted for themselves a way contrary to the one presented by the Qur'an, Allah sealed up their hearts and ears. The Qur'an simply states a law of Nature: if one takes a biased view of something and deliberately nourishes prejudices against it in his mind, he can neither see any virtue in it nor hear anything in its favour nor open his heart to consider it dispassionately. This is the law of Nature and, as it is Allah's law, the act of sealing up of the hearts and the ears and the covering of the eyes has been attributed to Him.