Two months to the day since fire engulfed Notre Dame Cathedral and destroyed much of its roof and vaulted ceiling, the archbishop of Paris has offered the first Mass in the charred building that is still in danger of partial collapse.

As work continues to stabilize the cathedral’s structure, Archbishop Michel Aupetit celebrated a Mass on Saturday, June 15, for a group of around 30 worshipers wearing white hard hats.

“It was a moment of emotion. Many of the people who attended work in the cathedral. They were touched. It’s their house,” Archbishop Aupetit told reporters after the Mass. “For believers, it’s a moment of hope. The cathedral will be back,” he added.

Many of those taking part in the Mass in Notre-Dame des Sept Douleurs—a chapel inside the cathedral that housed the Crown of Thorns, one of Christendom’s most important relics—were priests or canons. They stood close to a pile of burned debris and a number of yellow construction vehicles. At times, worshipers took off their hard hats to read from the Bible or receive Communion. The service was broadcast live by a Catholic television station.

“It’s a beautiful symbol,” French Culture Minister Franck Riester told local radio earlier on Saturday. “It will be for a small group of people and it’s understandable: There are still major security questions.” On Friday, Mr. Riester said the cathedral remains fragile and the vault could still collapse.

Firefighters took several hours to extinguish the blaze that started on April 15 and raged across the roof of the cathedral, toppling its spire. Since then, workers have been racing to stabilize the three facades that were attached to the now-absent roof. Between 60 and 150 workers are on site each weekday, and work to secure the structure isn’t expected to be completed for at least another three months, according to André Finot, a spokesman for the cathedral.

Saturday’s Mass celebrated the anniversary of the consecration of the cathedral’s altar, which usually takes place on June 16. Its aim was to “show the world that this place is a church, a place of faith, a place of worship,” said Karine Dalle, communications director for the Paris diocese.

“This cathedral is a place of worship, that’s its true and unique purpose,” Archbishop Aupetit said during the ceremony. “It’s important to remind people of that.”

The square in front of the cathedral and other areas around the building remain closed to the public. Authorities are still cleaning the area around the site and have issued health and safety guidelines for pregnant women and any children who frequent the area surrounding the building. Environmental campaigners have warned of the pollution caused by the fallout from hundreds of tons of lead melting when the tiled roof went ablaze. Paris authorities said last month that lead levels in the ground near Notre Dame were more than 30 times that recommended.

On Saturday, Mr. Riester, the culture minister, said around 10% of the €850 million ($955 million) pledged in funds to rebuild the cathedral had been paid. Several of France’s best-known companies and richest people, including luxury tycoons Bernard Arnault and François-Henri Pinault, made large pledges to the rebuilding fund in the days after the fire.

French lawmakers last month moved to fast-track the restoration of the cathedral, approving measures that could help President Emmanuel Macron achieve his goal of completing the task in the next five years. On Saturday, light streamed through a large hole where the roof used to be and several safety nets could be seen above the worshipers.

Mr. Riester said the government wouldn’t confuse “speed with haste” during the restoration.

“Of course, it’s the quality of the work that counts,” Mr. Riester said on Saturday. “It doesn’t mean that in five years the works will be finished in their entirety.”

A decision on whether to rebuild the cathedral’s spire as it was before the blaze or to add modern flourishes will be taken in a few months, the culture minister added. France will hold an international competition to design the spire, he said.

French authorities continue to investigate the cause of the fire.