The Jews rebel after Allah took Their Covenant and raised the Mountain above Their Heads
Allah reminded the Jews of their errors, breaking His covenant, transgression and defiance, when He raised Mount Tur above them so that they would believe and agree to the terms of the covenant. Yet, they broke it soon afterwards,
قَالُواْ سَمِعْنَا وَعَصَيْنَا
(They said, "We have heard and disobeyed.") We have mentioned the Tafsir of this subject before. `Abdur-Razzaq said that Ma`mar narrated that Qatadah said that,
وَأُشْرِبُواْ فِى قُلُوبِهِمُ الْعِجْلَ بِكُفْرِهِمْ
(And their hearts absorbed (the worship of) the calf) means, "They absorbed its love, until its love resided in their hearts." This is also the opinion of Abu Al-`Aliyah and Ar-Rabi` bin Anas. Allah's statement,
قُلْ بِئْسَمَا يَأْمُرُكُم بِهِ إِيمَـنُكُمْ إِن كُنتُم مُّؤْمِنِينَ
(Say: "Worst indeed is that which your faith enjoins on you if you are believers.") means, "Worse yet is the manner in which you behaved in the past and even now, disbelieving in Allah's Ayat and defying the Prophets. You also disbelieved in Muhammad , which is the worst of your deeds and the harshest sin that you committed. You disbelieved in the Final Messenger and the master of all Prophets and Messengers, the one who was sent to all mankind. How can you then claim that you believe, while committing the evil of breaking Allah's covenant, disbelieving in Allah's Ayat and worshipping the calf instead of Allah"
Tafsir Ibn Kathir (abridged, English).