There has been some speculation among fans as to the origin or etymology of the phrase. Some have claimed that it was engraved upon the sword of a legendary or historic knight or crusader. Others believe that it refers to the Biblical admonishment, "Let he who is without sin cast the first stone." Finally, some suggest that it is just a poetic login acknowledgment, left in the Megadeus operating system for forgotten reasons.

This expression has become a rather popular sig quote among anime fans of late -- possibly because of its very mystery. It's certainly more welcome in that capacity than, say, All Your Base.

Update: dragoon has pointed out that in the version of Big O shown in the U.S. by Cartoon Network, this line is redacted to "Cast in the Name of Good, Ye Not Guilty". Presumably this was done because someone thought that giant robots aren't supposed to talk about God, or some such ....

In fact, this log-in phrase shared among all the Big-class Megadeuses (those would be Big-O, Big Duo, Big Fau, and presumably Big Venus) does seem to have a purpose. When Alex Rosewater tried piloting Big Fao in the twenty-first episode, "The Third Big," after some initial success the controls froze up, and the robot began destroying things all on its own. On its screen it displayed the words, "CAST IN THE NAME OF GOD YE NOT," with a suggestive blank space where "GUILTY" would have been. It then yielded voluntarily to Big-O when it arrived, with Roger Smith at the controls.

More ominously, in the twenty-fourth episode, appropriately entitled "The Big Fight," Alex gave a rebuilt Big Duo to Alan Gabriel, who was hooked directly into its systems. He almost destroyed Roger and Big-O, gloating all the while, but eventually Big Duo seemed to get fed up with its diabolical pilot, displayed on its screen, "CAST IN THE NAME OF GOD YE GUILTY," and then... weird stuff happened. Wires poured out of the various hatches in the control panel, engulfing a screaming Alan Gabriel, apparently killing him, while a ghostly image of Schwartzwald, Big Duo's deceased previous pilot, looked on and laughed. In the next episode Big Duo was seen flying skyward, having ejected a mass of wires from its cockpit along with Alan's signature hat, and collided with... something high in the sky above the city. For more on that, check the node on The Big-O. (I'll be updating my writeup there later tonight with information on the show's second season.)