Seven and a half years after a raging fire nearly destroyed the Notre-Dame Cathedral in Paris, workers will soon begin installing the most contentious feature of its meticulous restoration: six towering stained-glass windows that depict the Christian feast of Pentecost.

Unlike the cathedral’s gargoyles, paintings and other treasures, which were refurbished, the windows are entirely new.

The work of a French artist, Claire Tabouret, they depict landscapes and traditional religious processions, but in a contemporary style. They will replace 19th-century stained-glass windows on the south side of the soaring nave, fueling a fierce debate over the ethics of introducing contemporary artwork to an 800-year-old masterpiece.

More than 300,000 people signed a petition demanding that the old windows be left in place, while preservationist groups filed multiple legal challenges against their replacement. They argued that it was a breach of France’s heritage laws, no small thing in a country that prizes its artistic and architectural patrimony.

“Demolishing the past, unraveling something that’s already designated as a historic monument, just to make way for something else — that’s pretty unprecedented,” said Julien Lacaze, the president of Sites & Monuments, a group that campaigns to protect France’s architectural heritage.

None of that resistance has been successful. The new windows are nearly complete and set to be installed by the end of 2026. Now, the officials overseeing Notre-Dame’s restoration are cautiously optimistic that, for all the earlier criticism, worshipers and visitors will ultimately welcome the latest addition to the cathedral, once they see it in place.

“It’s often like that,” said Philippe Jost, the president of the Public Establishment Rebuilding Notre-Dame de Paris. “Before, opinions are expressed, often in a passionate way, with the strength of conviction. Then, once things are there, people realize that in fact, they aren’t that bad.”

The windows are being made in a centuries-old glassmaking workshop in Reims, an ancient city 80 miles east of Paris. I visited it this summer with Mr. Jost, a tour that provided another twist in this tangled story. It turned out that the workshop, Atelier Simon-Marq, which was founded in 1640, is now tucked into the basement of a much younger church, the midcentury modern Église du Sacré-Coeur, or Church of the Sacred Heart.

The symbolism was hard to miss: A 1950s church, built of reinforced concrete and deconsecrated about 60 years after it opened, houses one of Europe’s oldest workshops. That atelier is in turn creating stained-glass windows for perhaps the world’s most famous gothic cathedral, the first stone of which was laid in 1163.

Inside the workshop, craftspeople hunched over light tables, painting and engraving pieces of cut glass that will be fired and assembled into the stained-glass windows.

The company was founded by Pierre Simon, a master glassmaker, and remained in his family’s ownership until nearly two decades ago. Over the centuries, it collaborated with artists like Marc Chagall and Joan Miró, and helped to restore the windows of Reims Cathedral after they were damaged in World War I.

After falling on hard times, the family sold the workshop in 2011. It was later sold again to two wealthy businessmen from Reims, Pierre-Emmanuel Taittinger, the former president of the Taittinger Champagne house, and Philippe Varin, a former chairman of the French automaker PSA Peugeot Citroën.

Mr. Taittinger, needing to move the workshop to larger quarters, hit upon the idea of putting it in a converted church. He said he thought this would provide a peaceful, almost monastic environment, consistent with restoring stained-glass windows.

Mr. Taittinger said he made his pitch to the newly installed archbishop of Reims, Éric de Moulins-Beaufort. “After one or two glasses of Champagne,” he recalled lightheartedly, “the bishop said, ‘Yes, I have one or two churches.’”

Later, Mr. Taittinger welcomed another archbishop, Laurent Ulrich of Paris, to the workshop. It was Archbishop Ulrich who first proposed replacing Notre-Dame’s 19th-century windows, which were designed by the French architect Eugène Viollet-le-Duc. The archbishop raised the idea with President Emmanuel Macron, who, eager to leave his mark on the restoration of Notre-Dame, embraced it.

After an international competition, the project was awarded to Ms. Tabouret, 44, a French artist who paints in a figurative style. She said she was intrigued, rather than put off, by the debate over installing new windows. “It’s not very usual that we do things like this in France,” she said. “That’s a very strong sign for the country, and for the church.”

Archbishop Ulrich, who suggested that Ms. Tabouret take Pentecost as her theme, defended the choice to go modern as entirely in keeping with the history of Notre-Dame. Since it was completed in 1345, he said, “the cathedral has been enriched, has undergone transformations and has welcomed them.”

Céline Bachelot, a manager at the Atelier Simon-Marq, said she was awed by the chance to create something that will become part of Notre-Dame. “It happens maybe once every two centuries,” she said.

Part of the optimism of the project’s backers stems from the positive reception for a recent exhibit of Ms. Tabouret’s full-scale sketches for the windows. Some 340,000 people visited the exhibit, at the Grand Palais museum in Paris, and the reviews in French newspapers were positive.

“My heart is beating a little bit,” said Ms. Tabouret of the year-end unveiling. “Obviously, some people won’t like them. You can’t please everybody when you have 15 million people a year visiting the cathedral.”

Ms. Tabouret, who recently returned to France after a decade living in Los Angeles, said she admired the passion that historical preservation aroused in the French. France, she said, was one of only a few countries where people could be overheard in a bar debating the merits of restoring old buildings.

But she dismissed the contention that her windows would erase the legacy of Viollet-le-Duc, whose windows were themselves part of a broader restoration of Notre-Dame. His windows will be restored and displayed: two at an architecture museum in Paris and four in a chateau north of Paris that he rebuilt.

And Viollet-le-Duc’s most visible contribution, the lead-covered wooden spire that rose above the cathedral’s transept, was rebuilt faithfully after it collapsed in 2019. French officials solicited cutting-edge proposals from prominent architects like Norman Foster before deciding to stick to the original design.

“If you go to the newly restored Notre-Dame, it’s an incredible homage to Viollet-le-Duc,” Ms. Tabouret said.

For now, though, she is more focused on finishing her own windows. In visits to the workshop every other week, she said she still tinkered with her designs. “I just changed some work on the faces,” she said in our interview.

“We’re working on the procession in the last window,” she said. “It’s the last image you see before you leave the cathedral.”

Reporting was contributed by Ségolène Le Stradic, Ana Castelain and Giulia Imbert.