A church on an island, a church on the Left Bank, and a church on the Right Bank — that's what I'm posting today. Whether you are religious or not (and many French people are not croyants or "believers"), you can't avoid churches in Paris. First, here's the preeminent French church, Notre-Dame de Paris. It's a cathedral, meaning that is is the headquarters of a Catholic bishop — not all big churches are cathedrals. This one was built over a span of two centuries, starting in the late 1100s.
In the photo of Notre-Dame above, you can also see a church in the background that is actually on the right (or north) bank of the Seine, sort of behind Notre-Dame's towers. It's the Eglise Saint-Gervais. In the foreground, you can see a church that is on the left (or south) bank of the Seine — L'Eglise Saint-Séverin is in the Latin Quarter, not far from the Sorbonne.
Below is a photo one of the major churches on the Left Bank of Paris. It's the Eglise Saint-Sulpice, in the Saint-Germain-des-Prés neighborhood. It's a much less ancient church than the ones in the photo above, having been built only in the 1600s and 1700s. Earlier, smaller churches had stood on the site for centuries, but a new one was needed to accommodate the growing population of the neighborhood.
Finally, below is a huge church on the Right Bank of Paris. It's called the Eglise Saint-Eustache, and the building has never really been finished. It stands on the edge of the old central market of Paris called Les Halles — [lay-AHL], with no pronounced S. Construction of the existing church started in the 1530s, during the French Renaissance.
The church's façade was redone in the 1700s. As you see, the south tower was left incomplete. Trivia: the powerful 16th century cardinal Richelieu, the great 17th century writer Molière, and the 18th century courtisane called Madame de Pompadour were all baptized at Saint-Eustache. The funeral of La Fontaine, author of fables, took place here in 1685.
And I lived for three years just a few hundred yards north, back in the late '70s and early '80s. I took the photos here from the top of the Tour Montparnasse.