Tafsir of Al-Ma'idah 5:78

(Those who disbelieved among the Children of Israel were cursed): Meaning, Allah the Exalted cursed them. The verb is constructed in the passive voice in accordance with the norms of majesty. The prepositional phrase relates to an elided term that serves as a circumstantial qualifier (hal) for the relative pronoun or for the agent of "disbelieved."

His saying—Exalted and Almighty is He—(by the tongue of David and Jesus, son of Mary): It relates to "cursed," meaning He—Majestic and Exalted is He—cursed them in the Psalms and the Gospel by the tongues of these two prophets (peace be upon them both), by His—Exalted is He—revealing within them: "Cursed is he who disbelieves from among the Children of Israel in Allah the Exalted or any of His messengers (peace be upon them)."

Al-Zajjaj said: The meaning is that David and Jesus (upon them both be peace) proclaimed the prophethood of Muhammad (may Allah bless him and grant him peace), gave glad tidings of him, commanded his following, and cursed those who disbelieved in him from among the Children of Israel. The first view is more appropriate, and it is what has been narrated from Ibn Abbas (may Allah be pleased with both of them).

It has been said: When the people of Eila transgressed regarding the Sabbath, David (upon him be peace) said: "O Allah, clothe them with the curse like a mantle and like a belt around the loins." Thus, Allah the Exalted transformed them into monkeys. Regarding the people of the Table, when they disbelieved, Jesus (upon him be peace) said: "O Allah, punish those who disbelieve after having eaten from the Table with a punishment that You have not inflicted upon anyone among the worlds, and curse them as You cursed the people of the Sabbath." Consequently, they became swine; they were five thousand men, among whom was neither woman nor child. This view is narrated from al-Hasan, Mujahid, and Qatada, and a similar report is narrated from al-Baqir (may Allah be pleased with him); it was chosen by more than one scholar. The "tongue" here refers to the bodily organ, and its singular form is one of the three well-known usages in such contexts. It is also said that it means "language."

(That): Meaning, the aforementioned curse. The preference for the demonstrative pronoun over the personal pronoun is to indicate the completeness of its manifestation and its distinction from its counterparts, aligning it—by virtue of it—in the sequence of observable affairs. The "ka" in dhalika (that) indicates distance, to signal the completion of its heinousness and the degree of its atrocity and terror.

(is because they disobeyed): Meaning, due to their disobedience. The prepositional phrase relates to an elided term that serves as the predicate for the preceding subject. The sentence is an inauguration that functions as an answer to what arose from the speech, as if it were said: "For what reason did that happen?" It was replied: That immense and heinous curse is because of their disobedience.

His saying—Exalted is He—(and they used to transgress): It is possible that this is conjoined to "disobeyed," and thus it falls under the scope of the cause; meaning, "and because of their continuous transgression." The combination of the past tense and the imperfect tense indicates the intention of continuity. Al-Zamakhshari claimed that the speech implies an exclusivity of the cause to what was mentioned—meaning, because of that and nothing else—as it was said that this is derived from the departure from the apparent structure, which would be the attachment of "because they disobeyed" to "cursed" without mentioning the demonstrative pronoun. When it was brought, it was to express disdain for that curse and to serve as an answer to the question of the cause, indicating that the totality of this [curse] is for this reason and no other. It is also said that it is derived from the causal particle itself, because what is immediately understood from it is the sufficient cause, and it implies that [exclusivity]. It is not a refutation of the exclusivity that their disbelief is also a cause—as implied by its inclusion in the relative clause—because what is mentioned within the causal scope here encompasses their disbelief as well.

It is also possible that it is an inaugural report from Allah the Exalted that their state and affair was transgression and exceeding the bounds in disobedience.