"And when God took a covenant from those who were given the Book, [saying]: You shall surely make it clear to the people," to the end of the verse. This is a covenant that God took from the scholars of the People of the Book: that they must explain to the people what is in their Book, which includes the [prophecy of the] Messenger of God and Islam. "But they cast it behind their backs and purchased with it a small price," meaning what they used to obtain of worldly gain in return for it. "And wretched is what they purchased," they purchased the Fire in exchange for Paradise.
﴿وَإِذْ أَخَذَ اللَّهُ مِيثَاقَ الَّذِينَ أُوتُوا الْكتاب لتبيننه للنَّاس﴾ الْآيَة، هَذَا مِيثَاق أَخذه الله على الْعلمَاء من أهل الْكتاب؛ أَن يبينوا للنَّاس مَا فِي كِتَابهمْ، وَفِيه رَسُول الله وَالْإِسْلَام ﴿فنبذوه وَرَاء ظُهُورهمْ﴾ وَكَتَبُوا كتبا بِأَيْدِيهِم؛ فحرفوا كتاب الله ﴿وَاشْتَرَوْا بِهِ ثَمَنًا قَلِيلا﴾ يَعْنِي: مَا كَانُوا يصيبون عَلَيْهِ من عرض الدُّنْيَا ﴿فَبِئْسَ مَا يشْتَرونَ﴾ اشْتَروا النَّار بِالْجنَّةِ.