Prior to this two-part story, Lee and Kirby had dropped a few hints as to provenance of the good professor, particularly his mysterious involvement with a Biblically-named villain, Lucifer, first seen in X-MEN #9. Lucifer doesn't figure into "Origin" directly, though when Xavier mentions having been the victim of a car-crash to the students hearing his story, one of them wonders if the crash led to Xavier's loss of his legs, and at this point Xavier first reveals that Lucifer was involved in that injury. In itself this reference was just foreshadowing for the story of Xavier's first encounter with that villain, revealed in issue #20. Yet the mention of Lucifer, the symbol of devilish pride, is appropriate to the story in which Xavier first speaks of his evil stepbrother, whose very name, Cain Marko, refers to a specific motif of Biblical myth.
What did the two comics-makers know or remember of the Bible's use of this motif? No one today can be sure, not even the still-living Stan Lee. I will guess that one of the creators vaguely remembered just one connotation of the "Mark of Cain:" that it was supposed to tell onlookers that Cain, slayer of his brother, should remain inviolate. This vague recollection probably became refracted through the superhero idiom to become a story about a man who possessed no mark as such, but who is invulnerable to harm-- and who has tried, on a previous occasion, to kill his brother.
The story proper begins as Xavier's mutant detector Cerebro issues a fateful warning of an impending menace approaching the school for mutants. He marshals his five young charges to set up defensive traps around the school, to slow down the attacker-- at which point Xavier tells the youngsters his origin-story, much as an old soldier, huddled in a bunker with young greenhorns, might reveal his personal tale in case of his death.
In contrast to the fairly flat origin-stories given to the five main heroes, Professor X gets a tale replete with a Freudian family romance, with the American "romance with the atom" tossed in. Xavier relates that when he was still a young boy, his scientist-father perished in a nuclear accident near the historically significant city of Alamagordo, New Mexico. Xavier's father, never seen and little more than a haunting presence of 'the good father," is mourned by the boy and his grief-stricken mother. However, one of the deceased scientist's colleagues, Doctor Marko, survived the accident that took the life of the young boy's father-- and right at the gravesite of Xavier's father, Marko takes the first steps toward courting the wealthy widow-- and he soon becomes Xavier's "bad father." Marko soon reveals himself as a fortune-hunter, but he visits no personal cruelties upon either Xavier or his mother-- except insofar as he brings his son Cain into the family. Cain introduces himself to young Xavier as the consummate bully.
Xavier's mother soon passes, leaving young Xavier stuck with two unwanted step-relations. However, the not-quite-thieves fall out when Cain tries to blackmail his own father, accusing him of having arranged for his colleague's death. Xavier intrudes on their argument, resulting in a fiery denouement that gives Marko one moment as a "good father."
The two youngsters remain chained together throughout college, during which time Cain becomes as invidious toward Xavier as his father was toward Xavier's father-- but Charles Xavier now possesses the skills of a psychic mutant, implicitly due to his father's exposure to atomic radiation, and this time, he easily trounces the piggish bully.
Cain's unreasoning hatred leads him to try to intimidate Xavier by recklessly driving the car they both occupy along a dangerous road at high speed-- which ends up being a displaced murder-attempt, when the car goes off the road. Good son Xavier saves both of them, though not without a great deal of painful trauma from his own injuries.
Finally, though the two stepbrothers separate as adults, they've brought together again, improbably serving in the same unit in the Korean War. Cain flees battle into a cave, and Xavier (whose name means "savior) tries to save him from himself. Cain comes across, and tampers with, a sacred ruby in the hidden temple of the god Cytorrak-- and this is what transforms him into the menace that is approaching the school in real-time.
Once Cain has become transformed into the unstoppable Juggernaut, a cave-in buries him, while Xavier escapes. Over a decade later, the Juggernaut tracks down Xavier, at last possessed of enough power to destroy his mutant brother.
The most mythic part of the Juggernaut story ends when Xavier finishes his Freudian backstory, and the rest of the story is largely one long fight-scene, as the X-Men repeatedly seek to beat down the nearly invulnerable villain. Only one last myth-motif appears in the end, for once the Angel deprives the villain of his protective helmet-- functionally, his Achilles heel-- brain once more defeats brawn, in a trope seen throughout the works of Lee and Kirby.
I would hardly be a devotee of Marvel not to mention the provenance of the name of the temple-god Cytorrak, for this was one of the many figures whose names had already appeared in the annals of Doctor Strange's adventures. Professor X's only comment on Cytorrak accords with a supernatural explanation for the deity:
Cytorrak is the most mysterious of all the deities of black magic! When he was finally driven from our own world, he left behind him the curse of the Juggernaut!While there's no knowing who came up with the name Cytorrak, I find it interesting that the deity's backstory-- which was never referenced in the Doctor Strange tales-- resembles the scenarios from some of H.P. Lovecraft's srories. Indeed, Lovecraft's alien divinity Cthulhu-- whose name slightly resembles that of Cytorrak-- also suffered a sort of exile from the world of men, though he presented the threat of returning, not leaving behind a curse. It's possible that the general motif of a deity's curse upon mankind was merged with that of God's protection of the sinner Cain.
Finally, I would be remiss to mention that in my opinion the name Cytorrak most resembles not Cthulhu, but *Sycorax* of Shakespeare's TEMPEST-- which just happens to include a brutish lout-villain and a struggle between rival brothers. But it's a theory that I'm sure cannot be validated or disproven...