Tafsir of Al-A'raf 7:76

(Then those who were arrogant said...) This is a new beginning, as has preceded. The relative pronoun and its clause were repeated instead of using a pronoun to signify that they uttered what they uttered by way of haughtiness and arrogance.

(We are disbelievers in that which you have believed in.) This is also a departure from the requirements of the apparent meaning—which would have been "we are disbelievers in that with which you were sent." The benefit of this, as some have said, is the rejection of what the believers had established as known and accepted as a given; it is as if they said, "That which you have made known and accepted is not of that nature."

It is stated in al-Intisaf that they deviated from that phrasing to avoid what might be implied by its apparent meaning—that they were affirming his messengership while they were denying it. This is not a place for mockery, unlike the statement of Pharaoh: "Indeed, your messenger who has been sent to you is mad." The purpose here is for each individual among the believers and the deniers to inform [the other] of their state. Therefore, the disbelievers purified their statement from any implication of belief in the messengership, as a precaution for their disbelief and as an exaggeration in their persistence.