A Conspiracy of Faith (2016) on Netflix

Plot summary

The letter inside the bottle is written in tar, and blood stains the paper. It reaches Department Q, where Carl Mørck, back from sick leave, reads it with Assad and Rose. The degraded words mention a bus stop, children locked up, and Jehovah, who will save them. Only a P of the signature survives, and growth on the bottle shows it drifted for six years. A school in Ballerup names two brothers, Trygve and Poul Holdt, who left for a religious revival trip. Then the Viborg police call: a child was pulled into a car in Skals, the village of the sect called The Lord's Disciples. There Magdalena and Samuel are missing, and their father says they are with an aunt in Sweden.