The truth of certainty will come when the disbeliever enters the hellfire and feels its heat. Allah (swt) says:

 وَأَمَّا إِن كَانَ مِنَ الْمُكَذِّبِينَ الضَّالِّينَ (92) فَنُزُلٌ مِّنْ حَمِيمٍ (93) وَتَصْلِيَةُ جَحِيمٍ (94) إِنَّ هَٰذَا لَهُوَ حَقُّ الْيَقِينِ (95) فَسَبِّحْ بِاسْمِ رَبِّكَ الْعَظِيمِ

(But if he was of the deniers [who were] astray, (92) Then [for him is] accommodation of scalding water (93) And burning in Hellfire (94) Indeed, this is the true certainty, (95) So exalt the name of your Lord, the Most Great.) (Al-Waqi’a: 92-96)

We have “knowledge of certainty” which is perceiving the image of hellfire, the “eye of certainty” in the hereafter when we pass on the bridge over the hellfire (Sirat) and see the hellfire by our eyes. As for (certainty of truth), it comes when the disbelievers are thrown in the hellfire and experience it.

Hence, does Allah’s (swt) saying: (it is the truth of certainty) in this verse under study, refers to the Holy Quran or the punishment which the disbelievers will experience? Without any doubt, the Holy Quran is the truth of certainty, however it may refer also to the regret of the disbelievers. Indeed, the truth of certainty is absolute and affirmed. The certainty here describes the Holy Quran that it is undoubtedly a revelation from Allah (swt); it is true free of falsity and certain without any doubt. So, the two words are mentioned together for affirmation.