I am sure that there are a great many churches in Paris to see and explore.  There are two however, that most people are familiar with. One is the famous Notre Dame and the other is the Sacré-Coeur.  Most people will visit the Notre Dame and look at the Sacré-Coeur from the distance when they get to the top of the Eiffel Tower or if they are in Le Marais where you can spot the church through the narrow streets.  Since we explored the Notre Dame last time we went to Paris, we went to the Sacré-Coeur today, as did the other half of Paris.  We happened upon a mass so we had to be discreet in taking pictures and certainly no panorama shots.  Between the Notre Dame and the Sacré-Coeur I prefer the latter I have to say.