An Exorcism by John Brown
When he was called, John sprinkled holy water. Youko-san, who took a few steps backwards with a frown, was caught from behind by Lin-san.
John lightly cut a cross.
“I accost thee, damned and most impure spirit, cause of malice, essence of crimes, origin of sins, thee who revel in deceit, sacrilege, adultery, and murder.”
John extended his hand towards Youko-san, who widened her eyes. He drew a cross towards Youko-san’s chest.
“I adjure thee in Christ’s name,”
He drew a cross towards her forehead.
“that, in whatsoever part of the body thou art hiding, thou shalt declare thyself, that thou shalt flee the body that thou hast occupied.”
“Thou shalt depart, wheresoever thou dost lurk, and nevermore seek out bodies dedicated to God.”
“Let these holy bodies be forbidden to thee forever, in the name of the Father, the Son, and the Holy Spirit.”
Towards her mouth. Then he held up his hand to gently touch her eyes.
Youko-san’s legs failed her and she collapsed. She slumped down on the spot supported by Lin-san. When Youko-san opened her eyes, which had been tightly closed, she wore a surprised expression.
John smiled at Youko-san, who was in dismay. He lightly kissed the cross he held in his hand and put it around Youko-san’s neck.
“You’re alright now. Please wear this and do not remove it.”
-Ghost Hunt, Volume 6 Chapter 4
I found the words from John's exorcism in the book Lucifer: The Devil in the Middle Ages by Jeffrey Burton Russell (pg 126). The author cites it as being from the Gallican liturgy (Gallican Rite), which is a historical version of Christian liturgy and other ritual practices in Western Christianity. The book was written in 1986 and is the closest version that I could find to the one Ono Fuyumi uses for John's exorcism.