PORTICI, Italy -- As Christmas approaches, cities and towns across Italy are getting dolled up with decorations. Not the town of Portici.

Nestled in the shadow of Mount Vesuvius, near the busy port of Naples, Portici is chock-full of Catholic faithful who will pile into local churches for Christmas Mass. But those who dare deck their streets and storefronts with boughs of holly risk running afoul of the law.

For...