"[Those] who assume [i.e., know] that they will meet their Lord."

Muhammad said: In the speech of the Arabs, 'zann' (assumption/thought) has two meanings: doubt and certainty. Durayd ibn al-Simma said:

(I said to them: 'Assume [know] with two thousand fighters... their elite, in Persian chainmail.')

The meaning of 'zannu' (they assumed/thought) here is: they were certain.