Translation:

23. Those who reject the Signs of Allah and the Meeting with Him (in the Hereafter) it is they who shall despair of My mercy: it is they who will (suffer) a most grievous Penalty.

Notes (Tafseer)

3443. The emphasis is on "they" (ulaika). It is only the people who ignore or reject Allah's Signs and reject a Hereafter, that will find themselves in despair and suffering. Allah's Mercy is open to all, but if any reject His Mercy, they must suffer.