You needn't be of strong faith to appreciate the peaceful atmosphere of Brandenburger Cathedral. Ask when the next mass is scheduled or when it's open for general viewing.
You'll have sightseeing choices galore when spending time in Brandenburg an der Havel. Brandenburger Cathedral is an ideal place to begin here, but there are loads of other fine sights to experience. Plauer See, Brandenburger Theater, and Malge are close to the principal church and will top off your day nicely.