Na Hong-jin has cited The Exorcist among the films behind this one, and the last act is where it shows: a child who is not a child any more, a father who has run out of secular explanations, and a rite performed by somebody whose faith is not sufficient. What The Wailing adds is the question of WHICH faith, staging a Catholic deacon, a shaman and a stranger against each other so the audience never learns which one was right.