It's no surprise that Juggernaut is easily one of the strongest X-Men characters in the Marvel Universe as he is powered by the magic of an ancient entity Cyttorak as opposed to the natural-born mutants he so regularly fights against. While his body is indestructible, the Juggernaut’s mind is still just as vulnerable as it would be if he were powerless. To combat psychic attacks, Juggernaut wears a helmet that is bolted to the rest of his armour, though more often than not he is dealing with someone with super strength who can rip it off, so Juggernaut has taken an extra precautionary measure.
Cain Marko first gained the powers of the Juggernaut during the Korean War where Marko served as a soldier. When under fire, Marko deserted to a cave where he found the lost Temple of Cyttorak and grabbed hold of a mystical ruby which imbued him Cyttorak’s raw power, giving birth to the Juggernaut. With his newfound powers, Marko became the arch-rival to his stepbrother Charles Xavier and to everything Charles held dear, including the mutant militant group the X-Men.
In Uncanny X-Men #218 written by Chris Claremont with art by Mark Sylvestri, Dan Green and Glynis Oliver, a few X-Men members are hoping for a nice day off when they encounter the destructive menace, the Juggernaut. Rogue, Psylocke, and Dazzler jump into action to stop the villain during his rampage. Dazzler blinds him with her light projection powers, Rogue steals Juggernaut’s strength to rip his helmet off, and Psylocke stands by to take him out psychically, but once the helmet is off, the X-Men run into an unexpected issue.
Showing his growth, Juggernaut has learned from past experiences with the X-Men and took an extra step in protecting himself by wearing a protective skull cap underneath his helmet. The added layer of mental protection gave Juggernaut enough time to counter the X-Men’s attack. Eventually, however, the X-Men are able to remove the second helmet and finally defeat the hulking villain. While the Juggernaut loses in the end, wearing the second helmet is a smart move, protecting his only vulnerability by taking steps to combat psychic attacks.
The Juggernaut identified basically his only weakness and made strides to ensure he is protected by adding another helmet underneath his main one. With that extra layer of protection, the X-Men only have two ways of defeating him, by either removing that helmet after the first as they had done in the past or by finding the virtually unprotected gem of Cyttorak and transferring Marko’s power from him and into someone else. While stealing the Juggernaut’s power would be a quick fix for handling Marko, the malevolent spirit of Cyttorak would just create a new enemy for the X-Men to fight. Leaving basically no other option in defeating him than a psychic attack, the X-Men are left with the option of removing Juggernaut’s helmet and beating him that way which is why the villain wears a helmet under his helmet.