(Those who reject Our Ayat, We shall gradually seize them in ways they perceive not) meaning, the doors of provisions will be opened for them and also the means of livelihood, in this life. They will be deceived by all this and think that they are on the correct path. Allah said in another instance,

فَلَمَّا نَسُواْ مَا ذُكِّرُواْ بِهِ فَتَحْنَا عَلَيْهِمْ أَبْوَابَ كُلِّ شَىْءٍ حَتَّى إِذَا فَرِحُواْ بِمَآ أُوتُواْ أَخَذْنَـهُمْ بَغْتَةً فَإِذَا هُمْ مُّبْلِسُونَ - فَقُطِعَ دَابِرُ الْقَوْمِ الَّذِينَ ظَلَمُواْ وَالْحَمْدُ للَّهِ رَبِّ الْعَـلَمِينَ

(So, when they forgot (the warning) with which they had been reminded, We opened for them the gates of every (pleasant) thing, until in the midst of their enjoyment in that which they were given, all of a sudden, We took them (in punishment), and lo! They were plunged into destruction with deep regrets and sorrow. So the root of the people who did wrong was cut off. And all the praises and thanks are to Allah, the Lord of all that exists.)6:44-45. Allah said here,

وَأَمْلَى لَهُمْ

(And I respite them) prolong what they are in,

إِنَّ كَيْدِى مَتِينٌ

(certainly My plan is strong) and perfect.