Tafsir of Al-Haqqah 69:51

"And indeed, it" refers to the Quran. It is "the truth of certainty," meaning the reality of certainty. The meaning is equivalent to the "essence of certainty," as it is on the same order as the essence of a thing and the thing itself. The genitive construction (idafa) here implies the letter lam (belonging to), as clarified in al-Kashf. It is also permissible for the genitive construction to imply the meaning of min (from), meaning: the truth derived from certainty. What has preceded in Surah al-Waqi'ah will benefit you here, so recall it.

Some of the Sufis—may their secrets be sanctified—have mentioned that the highest rank of knowledge is the "truth of certainty," below it is the "essence of certainty," and below that is the "knowledge of certainty." The first is like a rational person's knowledge of death when he tastes it; the second is like his knowledge of it upon witnessing the angels (peace be upon them); and the third is like his knowledge of it at all other times. The complete discussion regarding this should be sought in their books.