I guess it's some what sacred to that person. I as well don't know much but from context clues I can guess that much. Hope that help at least a little :)!
I don't know about it much either but according to the starting chapter of the manga the real hero (the knight of heroin) proposed the real heroin (her sis daughter) by telling her his baptismal name
It was stated baptismal name is very personal and usually only your close family and the person you marry know it.
That reminds me when Leslie got her Baptismal name. She only told it to her family and to the person she's close to.
can someone tell me ab baptismal names and why people don't say it since i don't really know much about church I've seen it in a lot of stories but just never understood why it's special??