At some point, the Spear was unearthed and/or recovered by the Nazis, as Adolf Hitler believed it had the power to make its bearer invincible. It was subsequently acquired by the Blackhawks, who decided to keep it safe in a vault on Blackhawk Island, along with various other weapons and pieces of advanced technology.
Yet Grodd, who was both a genius scientist and a firm believer in magic, concluded that the Spear had no powers and was ultimately worthless. For this reason, it amused him to test Lex Luthor's abilities, as a new member of the Legion of Doom, by sending him on a mission to Blackhawk Island to steal the Spear. When Luthor returned, with The Key and Dr. Polaris, Grodd remarked that he planned to decorate his office wall with it. The Spear was presumably destroyed when Darkseid blew up the Legion of Doom's Headquarters.
The Spear of Longinus, sometimes called the Holy Lance or The Spear of Destiny, was supposedly the soldier's spear that stabbed Christ at the Crucifixion. This religious background is not explicitly mentioned in the episode (see also List of "God" utterances in the DCAU).
The spear that appears in Justice League Unlimited seems, anachronistically, too primitive to have been a Roman soldier's weapon (not to mention impossibly well-preserved, for being almost 2,000 years old).
The Spear also features as a McGuffin in two recent films, Hellboy (2004) (based on the comic book series by Mike Mignola) and Constantine (2005) (based on the Hellblazer series partially owned by DC Comics). Both films mention Adolf Hitler's obsession with the occult, and link him to the Spear's re-discovery. The comic book miniseries Indiana Jones and the Spear of Destiny also featured the Spear as the MacGuffin and mentions Hitler's interest to acquire it before his death.