On The Traveling Professor small group tours to Paris, we usually visit some churches.   They have all different types of significance as noted below:

Cathédrale de Notre Dame de Paris (Notre Dame Cathedral)
Métro:  Cité or St-Michel Notre Dame.  Bus: 21, 24, 27, 38, 47, 85, 96.  RER:  B and C lines.
Over 10 million people visit the Cathedral of Notre Dame every year, so why shouldn’t you?  Admission is free to the cathedral itself.  If there is a line to get inside, it usually moves quickly.  However, there is usually a long and slow queue to climb the towers.  Opt to skip that part if time is short.  A visit to the cathedral on Île de la Cité is the perfect place to get your bearings and visit the center of Paris.  Behind the cathedral is the Pont Saint-Louis leading to the charming Île Saint Louis. 

Chapel of Our Lady of the Miraculous Medal
Métro:  Sevres-Babylone or Saint-Placide.  Bus:  39, 63, 70 84, 87, 94.
Practically hidden behind the Bon Marché department store at 140, rue de Bac, this is probably one of the most visited sites in Paris you never heard of.  Roman Catholic pilgrims flock to where the Blessed Virgin Mary appeared “in flesh and bones” to Saint Catherine Labouré in 1830 to request an oval medal be made, hence the Miraculous Medal. It is the Motherhouse of the Daughters of Charity of the Society of St. Vincent de Paul. Website:  www.chapellenotredamedelamedaillemiraculeuse.com.

Église Saint-Etienne-du-Mont
Métro:  Cardinal Lemoine.  Bus: 84, 89.
A cobble stone’s throw from behind the Panthéon, this Gothic church stands on the site of an abbey founded by Clovis.  It is dedicated to St. Genevieve, the patron saint of Paris.  This is a delightful place for a quick rest after visiting the Panthéon.  Admire the bright open space of this inviting church before heading onto rue Mouffetard.  The steps outside the church were used in Woody Allen's "Midnight in Paris" as Owen Wilson's character awaits to be picked up at midnight.  ½ hour.  Website (in French only): www.saintetiennedumont.fr.

Église Saint-Sulpice
Métro: Saint-Sulpice.  Bus:  63, 70, 86, 87, 96.
This is the 2nd largest church in Paris.  A brass meridian line is inlaid into the floor of the church and was recreated in a gripping scene from the movie The Da Vinci Code.  Catch the renowned organ recitals at the Sunday 10:30 a.m. mass. Visitors from all over the world come to hear organ virtuoso Daniel Roth.   ½ hour. Website: www.stsulpice.com.