You needn't have a strong religious faith to appreciate the calming atmosphere of Angouleme Cathedral. Look up when the next service starts or when it's open for general viewing.
You're spoiled for sightseeing choice when visiting Angoulême. Angouleme Cathedral is a good place to start here, but there are loads of other worthy sights to check out. Géant Casino, Chateau d'Angouleme, and The Comics Museum are near the principal church and will top off your day nicely.