Tafsir of Al-Ma'idah 5:78

"God revealed their curse in the Psalms" On the tongue of David, and in the Gospel on the tongue of Jesus. It is said that when the people of Ayla transgressed the Sabbath, David (peace be upon him) said: "O God, curse them and make them a sign." They were transformed into monkeys. When the companions of Jesus (peace be upon him) disbelieved after the [miracle of the] Table, Jesus said: "O God, punish those who disbelieve after eating from the Table with a punishment You have not inflicted upon anyone in the worlds, and curse them as You cursed the people of the Sabbath." They became swine—five thousand men, with neither women nor children among them.

"That is because they disobeyed" Meaning, this heinous curse, which was the cause of their transformation, was only for their disobedience and transgression, and nothing else. He then explained the disobedience and transgression by saying:

"They used not to forbid one another from the wrong they committed. How wretched was that which they were doing!" This is to express astonishment at the evil of their actions, emphasizing it with an oath. Alas for the Muslims in their turning away from the practice of forbidding wrong, and their lack of concern for it, as if it were not part of the religion of Islam at all, despite what they recite of God’s words and the exaggerations contained therein regarding this matter.

If you ask: How can the abandonment of forbidding wrong be an explanation for "disobedience and transgression"? I say: Because God Almighty commanded the forbidding of wrong; therefore, failing to do so is disobedience. It is a transgression because forbidding wrong cuts off corruption, so abandoning it is the opposite.

If you ask: What is the meaning of describing the wrong as "they committed it," when prohibition does not occur after the act? I say: It means they did not forbid one another from repeating a wrong they had committed, or from a similar wrong, or from a wrong they intended to commit—such as when you see the signs of engaging in immorality and its instruments being prepared and arranged, so you forbid it.

It is also permissible that it means: they do not desist or refrain from a wrong they have committed; rather, they persist in it and continue to do it. It is said: "He desisted (tanaaha) from the matter" and "He refrained (intaha) from it" when he abstains from it and leaves it.

"You see many of them..." These are the hypocrites among the People of the Book who were befriending the polytheists and showing them sincere affection.

"That the wrath of God is upon them" This is the specific object of the condemnation, and its place is in the nominative case (raf’), as if it were said: "How wretched is their provision for the Hereafter: the wrath of God upon them." The meaning is: "the cause of God's wrath."

"And if they had believed..." With a sincere faith, free of hypocrisy, they would not have taken the polytheists as allies.

"Allies" Meaning that befriending the polytheists is sufficient evidence of their hypocrisy and that their faith is not true faith.

"But many of them are defiantly disobedient" Rebellious in their disbelief and hypocrisy. It is also said the meaning is: If they had believed in God and Moses as they claim, they would not have taken the polytheists as allies, just as the Muslims did not befriend them.