Surah 27. An-Naml, Ayah 55

The ignorance referred to here is the spiritual Ignorance, the Ignorance of how grossness and sins that bring shame on their own physical and moral nature are doomed to destroy them: it is their own loss. That they knew the iniquity of their sins has already been stated in the last verse. That knowledge makes their spiritual Ignorance all the more culpable, just as a man consciously deceiving people by half-truths is a greater liar than a man who tells lies inadvertently.