Tafsir of Al-Furqan 25:6

(Say): [To them, as a refutation of them and to establish the truth]. (He has revealed it, He who knows the secret in the heavens and the earth): He, the Exalted, is described as having comprehensive knowledge of all hidden and manifest information—and by implication, all known things—to signal that what He has revealed contains secrets folded away from human intellects. Along with this, there is an insinuation regarding their disputations over their crimes, which are among His, the Exalted’s, knowledge. That is to say: it is not as you claim; rather, it is a celestial matter revealed by Allah, the Exalted, from whose knowledge nothing—not even the smallest thing—is absent. He deposited in it various forms of wisdom and secrets in a wondrous manner that intellects cannot grasp, as He has rendered you all collectively incapable through its eloquence and clarity, and informed you of future unseen matters and hidden things that cannot be guided to, nor accessed, except through the guidance of Allah, the Knowing, the Acquainted. If they intended by "morning and evening" that which is hidden from the people, the placement of the word "secret" becomes even more refined.

As for the conclusion with His, the Exalted’s, saying: (Indeed, He is Forgiving, Merciful):

It is to alert that they have merited punishment for the crimes mentioned, but it has been delayed for them because He, Glory be to Him, is eternally and perpetually characterized by forgiveness and mercy, which necessitate this delay. It is as if it were said: "He, Majestic and Exalted, is characterized by forgiveness and mercy as a matter of continuity; therefore, He does not hasten your punishment for what you are upon, despite your full merit for it and His, the Exalted’s, ultimate power to inflict it." Were it not for this, He would have poured the punishment upon you in abundance. Al-Tibi mentioned that, in this interpretation, there is a sense of astonishment, as in His, the Exalted’s, saying: "They have certainly become arrogant within themselves and become insolent with great insolence."

It is also permitted that the speech be a metonymy for His great power to punish them, for none is described as Forgiving and Merciful except one who is capable of punishment. In selecting these attributes, there is a reprimand for them and a condemnation of their actions; meaning: "You are in a state such that one whose attribute is Forgiveness and Mercy is [still] addressing your punishment," and that is no small matter. The author of al-Fara'id said: It may be said that the mention of forgiveness and mercy after that is so that they might know that these great, boundless sins are forgiven if they repent, and that His mercy will reach them thereafter, and so that they do not despair of the mercy of the Exalted regarding what they have committed, despite their persistence in the extreme hostility and disputation they are upon. And it is as you see.