Slayer guitarist Kerry King got the idea for this song while walking around Athens, thinking about all of the clashes that took place there over the centuries. "It's just about battle, not a particular battle," he told Rolling Stone. "You cast the first stone, I'm gonna win."
The phrase "To cast the first stone" means to take the lead in fault-finding or the like. The term is from The Gospel of John 8 v7, which tells of Jesus criticizing the Jewish religious leaders who wanted to pass judgment on an adulterous woman.
"He that is without sin among you, let him first cast a stone at her."
New Jersey hardcore punk band Ensign's 1999 album was also titled Cast the First Stone.