The mythical Greek hero Achilles was thought to be immortal. He won many great battles in the Trojan War, yet he was defeated because of his one area of vulnerability – his heel. There had been a prophecy that Achilles would die young, and therefore his mother Thetis dipped her baby son in the river Styx in an attempt to make him immortal. She held him by his left heel, and thus the area where she held him did not touch the water. He was therefore killed by his opponent Paris on the battlefield, by a well-aimed arrow in his heel the only part of him that was not immortal.
It made no matter how mighty Achilles was, he was still brought down by a very small defect. Likewise each one of us can have a weakness in the soul, our own Achilles heel that can one day become our downfall.