When visiting Paris by yourself or on a small group tour of Paris with The Traveling Professor, these religious sites are worth looking into:

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.
Museum Pass: Only needed for towers.
The cathedral is a classic example of 12th century Gothic architecture.  It stands on Île de la Cité and is close to Sainte-Chapelle and the Conciergerie.  To climb to the top of the cathedral, arrive early, before 9 a.m.  There is no admission fee to get into the cathedral itself.  The Museum Pass doesn’t permit skipping the line to climb the towers.  Bring binoculars to view the stained glass and other objects.  Visit the “Treasury” inside the cathedral for a small admission charge.  On display are holy relics such as crucifixes and gifts by Popes. The Crown of Thorns, worn by Jesus, is put on view every Friday during Lent and on the 1st Friday of each month.  Île Saint-Louis is directly across the bridge from Notre Dame.   This is where I always start my trip in Paris.  In the center of the “Parvis” (yes, the spelling is correct) or the plaza in front of the cathedral, notice the bronze marker from which all distances in France are measured.  Be vigilant of pick-pockets and scammers.  1-2 hours.  Website:  www.notredamedeparis.fr.

Basilique de Saint-Denis (Cathedral Basilica of Saint-Denis) 
Métro:  Basilique de St-Denis Museum Pass: Yes.
It’s full of history, some of it strange, and lots of dead people you learned about in history books. It is the burial place of nearly all the kings and queens of France, although the story is not quite that simple.   Visitors are fascinated by the tombs of people like Clovis, Marie Antoinette, and Louis XIV.  It is a 30-40 minute métro ride from the center of Paris and takes about 90 minutes to visit.  Check the website for last-minute changes to visiting hours:  www.saint-denis.monuments-nationaux.fr.

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.  ½ hour.  Website (in French only): www.saintetiennedumont.fr.

American Cathedral in Paris
Métro:  George V or Alma Marceau.  Bus:  32, 92.  
Museum Pass: N/A
Since 1886, this vibrant and lovely cathedral has served as a center of worship for English speakers in Europe.  It is under the direction of the Episcopal Church.  Inside, the flag of each state of the United States is flown.  The cathedral maintains an active arts and social program – excellent for those looking to meet other English speakers.  About 30 minutes.  Located at 23, avenue George V.  Website: www.americancathedral.org.

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.