You needn't be religious to enjoy the calming atmosphere of San Pedro Cathedral. Ask when the next service starts or when it's open for general viewing.
You're spoiled for sightseeing choice when in Soria. San Pedro Cathedral is an ideal place to kick off here, but there are lots of other interesting places to consider. Numancia, San Saturio Hermitage, and Palacio de Los Condes de Gomara are near the principal church and will round out your day nicely.