What is the meaning of the verb "kāna" (كان) in this verse?

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

1. It means Iblis "became" one of the disbelievers. [Qatāda]
2. It signifies the past tense, meaning: “In Allah’s knowledge, he was always a disbeliever.” [Muqātil, Ibn al-Anbārī]