Tafsir of Ad-Dhariyat 51:5-6

"Indeed, that which you are promised is true. And indeed, the Recompense is to occur."

This is the response to the oath. The ma (in innama) is relative (mawṣūlah), and the resumptive pronoun (al-ʿāʾid) is omitted—meaning: "Indeed, that which you are promised," or "that which you are promised of." It is also possible that it is an infinitive (maṣdariyyah), meaning: "Indeed, your promise or your threat [will be true]."

Regarding the verb tūʿadūn (you are promised), it may be the present tense of waʿada (to promise), or it may be the present tense of awʿada (to threaten). Perhaps the latter is more appropriate given the Almighty’s saying: "So remind by the Qur’an him who fears My threat," and because the purpose is intimidation and terror. It is reported from Mujahid that the verse refers to the disbelievers, which supports the interpretation of a threat. The meaning of its truth is the certainty of its occurrence.

In al-Kashshaf, it is noted as "a truthful promise," similar to [the linguistic construction of] "a pleased life" (ʿīshah rāḍiyah).

Al-Dīn refers to the recompense (al-jazāʾ), and its occurrence refers to its actualization. The majority [of scholars] hold that the promised event is the Resurrection. In specifying the aforementioned [entities sworn by] in the oath, there is a symbolic indication of their testimony to the certainty of the matter sworn upon, in that they are wondrous affairs; thus, He who is capable of them is capable of actualizing the promised Resurrection.