Say “Holy Spirit,” and most imagine a dove or tongues of fire gently descending. But the Celts envisioned something wilder: a goose in flight—honking across the sky, untamed and impossible to ignore.

You’ve heard of a wild goose chase, right? It’s a hopeless pursuit, a foolish search that is certain to prove unsuccessful. The adventurer who sets out on a wild goose chase wastes a lot of time ...