Shock and horror — he found his flock secretly flocking to a sacred tree with a golden snake. So he went in for prophecies to win them back.
He prophesied Emperor Constans II would besiege the town. Which he did. The next prophecy was that if the flock returned the siege would be lifted. They did. It was.
So the Bishop chopped down the tree and melted the golden snake to make a chalice.