‘If they believe’ means ‘if they believe in their opinion’...
‘If they believe’ means ‘if they believe in their opinion’, it is about the People of the Book, that is, the Jews and the Christians and what is found in the explanation of the of the traditions is more convincing than their explanation because ‘that which had been revealed to us ’ is more fitting with this Tafsir as Quran was first revealed to the Holy Prophet (S) and to their Ahlul Bayt (a.s.) who were present in the house of revelation and thereafter it reached other people and with it has been mixed (joined) what had been revealed to Ibrahim and Ismail and to other Prophets.
Therefore, as there is the description in these two phrases of the Prophets and Messengers it is fitting with this (revealed to us) also because the grade of ‘towards them’ should be like the messengers and their legatees.
Kulaini and Nomani say that when the explanation of the verse: And there are some among men who take for themselves objects of worship besides Allah, whom they love as they love Allah … (Sura Baqarah 2:165 ) was inquired from Imam Baqir (a.s.) he said: They are the first and the second whom people made their Imams leaving the one whom Allah had made Imam for the people.
And O, that those who are unjust had seen, when they see the chastisement, that the power is wholly Allah’s and that Allah is severe in requiting (evil). When those who were followed shall renounce those who followed (them), and they see the chastisement and their ties are cut asunder. And those who followed shall say: Had there been for us a return, then we would renounce them as they have renounced us.
Thus will Allah show them their deeds to be intense regret to them, and they shall not come forth from the fire. (Sura Baqarah 2:165-167) It is said that: By Allah, they are the leaders in oppression those who had led oppression by grabbing the rights of Ahlul Bayt (a.s.) and those who obeyed the grabbers.