UMM (╯°Д °)╯╧╧ HERE U GO!!
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. He also has the
Demon king's heart inside of him...The pillar awoke a part of its power thats why the holy sword rejects him.
<spoiler>
Here it is and this is what made me love claude even more. I read these spoilers a long time ago so i’m just going by memory here. Someone feel free to correct me if i get something wrong.
So the reason he has the demon king’s heart is because he was the one who turned back time. Dun dun dunnnn~
He loved ivona even during the first timeline so when he found out she had died, he went to the church to find a way to bring her back. Turns out tho that the church was shady af. They granted him his wish to save ivona but in return he has to eat(?)/be the host for the demon king’s heart so he can be resurrected.
Omg, I feel so anxious about that FREAKING sword do you guys have good spoilers, 'cause I'm about to blow shit up (╯°Д °)╯╧╧