User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-104.7.80.176-20150113124408/@comment-1359322-20150113125124

Because he's Yamamoto Genryuusai reincarnate. Just kidding.

Exorcists are just trained since they're young. The Church will select people with strong beliefs and give them special training at their institution. There are also institution that does human research.