"Decades ago, a wizard like us, too strong for his own good, rose from ignominy and tried to wrest power from the Ministry. His name was Grindelwald. He was swiftly captured and killed by an army of Aurors, but he got dangerously close to the seat of power.  Since then, the Ministry has taken care to prevent such disasters from ever happening again. They make sure that young and gifted children who might "go astray" never learn how strong they are. They keep us in the dark."