The Jews rebel after Allah took Their Covenant and raised the Mountain above Their Heads
Allah says;
وإذ أخذنا ميثاقكم ورفعنا فوقكم الطور خذوا ما آتيناكم بقوة واسمعوا
And (remember) when We took your covenant and We raised above you the Mount (saying), "Hold firmly to what We have given you and hear (Our Word).''
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.
قالوا سمعنا وعصينا
They said, "We have heard and disobeyed.''
We have mentioned the Tafsir of this subject before.
وأشربوا في قلوبهم العجل بكفرهم
And their hearts absorbed (the worship of) the calf,
Abdur-Razzaq said that Ma`mar narrated that Qatadah said that it,
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,
قل بيسما يٲمركم به إيمانكم إن كنتم مومنين
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"