You needn't have a strong religious faith to enjoy the calming atmosphere of Valdivia Cathedral. Ask when the next service is scheduled or when it's open for general viewing.
You'll have sightseeing choices galore when staying in Valdivia. Valdivia Cathedral is a good place to begin here, but there are many other interesting spots to consider. Valdivia Square, Cervecería Kunstmann, and Rio Cruces Bridge are close to the principal church and will round out your day nicely.