And if their evasion is difficult for you, then if you are able to seek a tunnel into the earth or a stairway into the sky to bring them a sign, [then do so]. But if Allāh had willed, He would have united them upon guidance. So never be of the ignorant.
(We know indeed the grief which their words cause you;) meaning, We know about their denial of you and your sadness and sorrow for them. Allah said in other Ayat,
فَلاَ تَذْهَبْ نَفْسُكَ عَلَيْهِمْ حَسَرَتٍ
(So destroy not yourself in sorrow for them.) 35:8, and
About the statement: فَإِنَّهُمْ لَا يُكَذِّبُونَكَ (it is not you they are belying, rather, it is the signs of Allah they are denying) in the first verse (33), there is a related event, reported in Tafsir Mazhari on the authority of a narration by As-Suddiy, according to which, once Akhnas ibn Shuraiq and Abu Jahl, two chiefs of the Quraysh tribe met together. Akhnas asked Abu Jahl: 0 Abul-Hikam, (In Arabia, Abu Jahl was called Abul-Hikam - 'man of wisdom' - but because of his hostility to Islam, he became known as Abu-Jahl - 'man of ignorance' ), you and me are alone here. There is no third person listening to us. Tell me about Muhammad ibn ` Abdulluh. Tell me what you really think about him. Is he telling the truth or is he not?
A life lived on the assumption that the world belongs to God is a truly God-fearing life, i.e. a life of taqwa, whereas a life in which no such assumption is made is one of sinfulness, being entirely given over to worldly pleasures and trivialities. The latter pleasure-seeking life will last—in terms of eternity—for only a matter of days and, with death, will come to an end, whereas the God-fearing life, regulated by God’s eternal principles, will continue to stand man in good stead both in this world and the next. In the present world man denies these realities but, as soon as the freedom given to him under test conditions ends, he will be compelled to accept them, though acceptance at that time will be of no avail.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.