You needn't be of strong faith to enjoy the peaceful atmosphere of Oulun Tuomiokirkko. Ask when the next service starts or when it's open for general viewing.
You'll be spoiled for sightseeing choice when spending time in Oulu. Oulun Tuomiokirkko is an ideal place to start here, but there are lots of other fine sights to discover. Nallikari, University of Oulu, and Oulu Market Square are near the principal church and will fill in your day nicely.