What is the meaning of the phrase "in kuntum mu'minin" in this āyah?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. The particle in has the meaning of negation, hence: "You were not believers since you disobeyed Allah and worshiped the calf."
2. It has the meaning of "if", forming a conditional clause which links to what precedes it, meaning: "If you were believers, then how evil is that belief which commands you to worship the calf and kill the prophets."

Both opinions were mentioned by Ibn al-Anbārī.