Every Sunday, about two dozen people gather at a green cabin along the main drag of Big Bear, Calif., a small mountain town known for its namesake lake. They go there for Jah Healing Church services, where joints are passed around.

April Mancini, a founder of the church, said she was drawn to the idea of cannabis as a religious sacrament back in 2013, after she met a Rastafarian who was running the place as an unlicensed medicinal dispensary.

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Defining what counts as a religion under the law “has been a notoriously difficult question for the courts ever since the founding,” said James Sonne, a Stanford Law School professor and the director of the Religious Liberty Clinic at the university.

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Lawyers for cannabis churches are arguing that marijuana is a sacrament that must be dispensed by religious institutions to ensure that the sourcing and the blessing of the product meets their standards. The courts must determine whether those practices are “analogous to mainstream faiths in terms of moral duty, ultimate concern, comprehensiveness, that sort of thing,” Mr. Sonne said. The court also must determine whether the belief system in question is “sincerely held.”