You needn't have a strong religious faith to appreciate the tranquil atmosphere of St. Georges Cathedral. Ask when the next mass starts or when it's open for general viewing.
You'll be spoiled for sightseeing choice when in Jerusalem. St. George's Cathedral is a great place to begin here, but there are plenty of other worthwhile places to enjoy. Western Wall, Dome of the Rock, and Temple Mount are close to the principal church and will round out your day nicely.