Tafsir of Al-Qasas 28:86

(And you were not expecting that the Book would be cast to you): This is also a confirmation of that, meaning: He will return you to the place of return just as He revealed the glorious Book to you, when you were not expecting it. Abu Hayyan and al-Tabrisi said: This is a reminder of His blessing—Mighty and Majestic is He—upon him, peace and blessings be upon him.

His saying: (Except as a mercy from your Lord), according to what al-Farra’ and a group [of scholars] have held, is an interrupted exception (istithna’ munqati’), meaning: But rather, He—Exalted is He—cast it to you as a mercy from Him, Mighty and Majestic is He. It is also permitted that it be a connected exception (istithna’ muttasil) from the most general of causes or the most general of states, on the basis that the intent is to negate the casting in the most emphatic manner. Thus, the meaning would be: The Book was not cast to you for the sake of anything except for the sake of mercy, or in any state except in the state of mercy.

(So do not ever be an assistant to the disbelievers): Meaning, do not be a supporter for them in their religion. Muqatil said: The disbelievers of Mecca invited him—may Allah Almighty grant him peace—to the religion of his forefathers, so Allah Almighty reminded him of His blessings and forbade him from assisting them in what they were upon.