誓願皈依 He Vows to Take Refuge
那郡侯磕頭禮拜，誓願皈依。當時召請本處僧道，啟建道場，各各寫發文書，申奏三天。那郡侯磕头礼拜，誓愿皈依。当时召请本处僧道，启建道场，各各写发文书，申奏三天。
逐句解 Line by line
The prefect knocked his head on the ground in reverence and vowed to take refuge.
磕頭 is the kowtow, touching the forehead to the floor — the word English borrowed. 誓願 is a vow in the religious sense, the same word used of a bodhisattva's undertaking to save all beings.
There and then he summoned the local monks and Daoist priests, opened a ritual ground, and had documents written out and sent up, one from each, to report to the Three Heavens.
道場 takes chǎng — a genuine place, a ritual ground — where the measure word for an event takes cháng. This chapter uses both readings of the character. 三天, the Three Heavens, is a Daoist arrangement of the cosmos, so the prefect is petitioning both religions at once and covering himself accordingly.
The conversion is instant and it is administrative. He kowtows, he vows — and then he summons clergy, opens a ritual ground and has the paperwork drafted.
This is a man doing the only thing he knows how to do well. He has spent the chapter being told that heaven runs on documents, and his response to a spiritual crisis is to file. That it is also sincere is exactly the point: the novel does not treat the two as opposites.
Both religions get a document. He is not taking chances.