If I'm not mistaken I think ML has a dark attribute so that's why he couldn't handle the holy sword but his parents force him so he had to force himself to get used to holding the holy sword. Everytime he hold the holy sword, he will feel an electronic shock but he endure it even thou it's still painful. So that's what I remember, Pls correct me if I'm wrong.
the ML was the chosen whatever, but the holy sword rejected him and they did indeed force him to hold it even when he was getting hurt bc of the rejection, but later on the sword accepted him somehow, and he stopped getting injured for a while. but after going to the ruins, it started rejecting him again

I forgot the reason why he cant hold the holy sword. Can someone please enlighten me? Thank you!