ummm maybe dangerous is not the words for it... it's something like scared(?) The ghost/spirit is scary, like when you're watching horror movie and then scared at the presence of the ghost, something like that maybe(?) So probably the priest are just scared I think. Also in this manhwa mage and priest are sensitive to spirit like being I think...if I remember right...but don't worry the MC are too OP and even he then grabbed the ghost hair like it was nothing. You can read chapter 86 and 87 of the novel for more details (https://www.readlightnovel.org/m-e-m-o-r-i-z-e/chapter-86) it's been long time since I read it, probably I missed something while I explained it