Final answer:
The main idea of the passage is the risk of becoming prideful and vulnerable after a victory, which can lead to further defeats due to a loss of vigilance and humility.
Step-by-step explanation:
The main idea expressed in the passage is the danger of becoming complacent and prideful after a victory, which can lead to being unprepared for future challenges. The text warns that when we focus too much on our success, we tend to forget the source of our achievements, namely the guidance and gifts given by a higher power, or in a secular interpretation, the fundamental factors that led to success. This can result in a loss of humility, an increase in pride, and a state of vulnerability to new attacks or failures. The passage uses the metaphor of an Achilles heel to illustrate the point that self-congratulatory behavior can expose our weaknesses and lead to our downfall.