You needn't have a strong religious faith to enjoy the serene atmosphere of Puerto Montt Cathedral. Ask when the next mass is scheduled or when you can go for a wander through.
You'll have sightseeing choices galore when spending time in Puerto Montt. Puerto Montt Cathedral is a good place to start here, but there are plenty of other fine places to experience. Angelmo Fish Market, Mall Paseo Costanera, and Plaza de Armas are not far from the principal church and will round out your day nicely.