(… فَلَمَّا نَسُوا مَا ذُكِّرُوا بِهِ) (So when they forgot that by which they had been reminded, …): The Messengers had come to them with reminders, giving them the good tidings of Paradise and warning them of the punishment of the Hellfire. These Messengers reminded them of the innate disposition of faith that they had forgotten from the previous messages.

(… فَتَحْنَا عَلَيْهِمْ أَبْوَابَ كُلِّ شَيْءٍ… ) (…We opened to them the doors of every [good] thing …): Why is this so? This is because, the human being – as the saying goes – will not fall from a straw mat. So, the doors of everything were opened for them, such that they were deluded by their wealth, status and health. Here, the prepositional phrase (opened to them) indicates that the doors of wrath and afflictions were opened for them, and not the doors of blessings. However, they did not realize this. In this verse, Allah (swt) reminds creation regarding blessings and afflictions. He might grant blessings as warnings, but He might also give afflictions so that the human being returns to the correct path.

(  حَتَّىٰ إِذَا فَرِحُوا بِمَا أُوتُوا أَخَذْنَاهُم بَغْتَةً فَإِذَا هُم مُّبْلِسُونَ … ) (…until, when they rejoiced in that which they were given, We seized them suddenly, and they were [then] in despair): They become hopeless and desperate. So, when the doors of everything had been opened to them, and they had forgotten the reminders they had received, had been deluded by the worldly life, and Satan had adorned falsehood for them, they become prideful of what they have been given. Allah (swt) then seized them by surprise, thus they fell into despair and hopelessness.