"Earthen vessels" refers to objects made of clay or pottery. In a metaphorical sense, it symbolizes human beings who are considered fragile, imperfect, and prone to mistakes. Full definition
Matthew Lee Anderson is the author of Earthen Vessels: Why our Bodies Matter to our Faith and The End of our Exploring: A Book about Questioning and the Confidence of Faith. (rachelheldevans.com)
In Keillor's case, we sense that he looks at the discrepancy from both sides, appreciating not only how earthen a vessel the church is, but how precious is the treasure it's trying to hold. (religion-online.org)