What does the pronoun "it" refer to in "don't be the first to disbelieve in it"?

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

1. It refers to what has been revealed (of the Qurʾān). [Ibn Masʿūd, Ibn ʿAbbās]
2. It refers to what is in their possession, meaning that if they conceal the description of the Prophet Muḥammad (ﷺ) while they possess it, they would be guilty of disbelieving in it. [Mentioned by al-Zajjāj]