Possession of books denounced as heretical was made a criminal offense. Copies of such books were burned and destroyed. But in Upper Egypt, someone, possibly a monk from a nearby monastery of St Pachomius, took the banned books and hid them from destruction - in the jar where they remained buried for almost 1,600 years.
Elaine Pagels (2004). “The Gnostic Gospels”, p.17, Random House