إِنَّ الْإِنْسَانَ لَفِي خُسْرٍ
Indeed, humankind is surely in loss. (العصر: 2)
The verse serves as the response to the oath. Inna governs al-insāna as its noun; the predicate includes la-fī khusrin, in which the lām adds further emphasis. The oath, inna, and lām combine to underscore the seriousness of the statement.
This reinforcement need not prove that every listener is an active denier. Emphasis can reflect the gravity of the message, heedlessness, or expected resistance. The stated generality is subsequently delimited by the exception, so analysis must take the next verse into account.