Karl Glogauer is an aspect of the Eternal Champion. He is a time traveler who came from the modern day to the ancient Holy Land in order to find and speak to the real Jesus Christ. This set in motion an ironic and unfortunate chain of events in which Karl himself was crucified under the name of Jesus, creating a stable time-loop and becoming the actual inspiration for the messiah he sought to find.
It is possible Karl was still a messiah of some kind, as, historically, his blood filled the Holy Grail. This blood would have been the Champion's own blood, and might have imbued the Grail with cosmic power.