The Quran

وَلَئِن أَذَقناهُ رَحمَةً مِنّا مِن بَعدِ ضَرّاءَ مَسَّتهُ لَيَقولَنَّ هٰذا لي وَما أَظُنُّ السّاعَةَ قائِمَةً وَلَئِن رُجِعتُ إِلىٰ رَبّي إِنَّ لي عِندَهُ لَلحُسنىٰ ۚ فَلَنُنَبِّئَنَّ الَّذينَ كَفَروا بِما عَمِلوا وَلَنُذيقَنَّهُم مِن عَذابٍ غَليظٍ

Muhammad Asad

Yet whenever We let him taste some of Our grace after hardship has visited him, he is sure to say, "This is but my due!" and, "I do not think that the Last Hour will ever come:¹ but if [it should come, and] I should indeed be brought back unto my Sustainer, then, behold, the ultimate good awaits me with Him!"² But [on the Day of Judgment] We shall most certainly give those who were bent on denying the truth³ full understanding of all that they ever did, and shall most certainly give them [thereby] a taste of suffering severe.⁴

I.e., man is, as a rule, so blinded by his love of this world that he cannot imagine its ever coming to an end. Implied in this statement is a doubt as to whether there will really be an afterlife, and whether man will really be judged by God on resurrection.
Being fully convinced of his own merit (as expressed in the words, "This is but my due"), he is confident that - in case there should really be a life after death - his own flattering view of himself will be confirmed by God.
I.e., the truth of resurrection and of God's judgment.
I.e., the realization of the spiritual blindness in which they spent their life will in itself be a source of their suffering in the hereafter: cf. 17:72 - "whoever is blind [of heart] in this [world] will be blind in the life to come [as well]".