
When Claude absorb that power from that entity in the ruins, it was dark magic. The holy sword was a gift from their god with pure and light magic power. The holy sword rejects when its wielder isn't selfless with a pure mind and heart. So the sword wouldn't accept Claude who was engulfed in dark magic with intentions of harming others to protect one, Ivona. Light and darkness are opposites so one can't be used when the other is in effect. Claude is able to go between the two magics like an on/off switch.

I think so . as mc mentioned demon king was never found in her previous life. Maybe because ml don't had anyone to protect so he doesn't became demon king but now he has mc to protect so maybe his powers are coming back and I don't think holy sowed ever
Excepted him in the first because when he was a kid it always rejected him.i don't know what happened after that.
soooo does anyone know exactly why the holy sword is rejecting him?