You needn't be religious to enjoy the calming atmosphere of Brazzaville Cathedral. Ask when the next mass is scheduled or when it's open for general viewing.
If you're staying a while in Brazzaville, Brazzaville Cathedral is a fine place to kick off your sightseeing. This principal church is one of many attractions that'll keep you on the go here. Ecole de Peinture de Poto-Poto, Presidential House, and Basilique Sainte-Anne are also waiting to be discovered nearby.