Commentators explain that Manoah's terror was a common and biblically-rooted belief in the Old Testament. Based on passages like Exodus 33:20 ('no man shall see me and live'), it was understood that a direct encounter with a manifestation of God would be fatal for a sinful human. Manoah correctly perceived that he had seen a divine Person, not just a created angel, leading to his cry, 'We shall surely die.'