User blog comment:B214/General Discussion/@comment-29963364-20161223084324/@comment-9595671-20161227191404

That's because the True Longinus killed him, it gets it's name over the fact Saint Longinus, the man who stabbed him to death, was the one who did the deed.

It's also pretty much the sole reason Longinus are classified as Sacred Gears that have the power to kill gods. It may have been the only one to actually do it, but helped set the standards for the potential.