When the Ursuline nuns learned that Andrew Jackson’s forces had prevailed over the British on the Chalmette battlefield on Jan. 8, 1815, their mother superior made what Sister Donna Hyndman today describes as a “rather rash’’ promise.

She said the nuns, who had held an all-night vigil to implore the help of Our Lady of Prompt Succor for the American forces in the Battle of New Orleans, would hold a thanksgiving Mass on that day — in perpetuity.

Mother Ste. Marie Olivier de Vezin’s spiritual descendants, including Hyndman, have honored her pledge ever since. And when the bicentennial of the Battle of New Orleans arrives in two months, the Mass that commemorates that seemingly miraculous victory will be celebrated in a shrine that has undergone what the sisters see as a miraculous transformation in its own right.

Three years ago, Sister Carla Dolce, prioress of the Ursuline Convent, was faced with a serious challenge. The shrine on their State Street campus, built in 1928, was deteriorating, with a badly leaking slate roof, an entrance that was poorly lit and hard to navigate, and an interior that was dingy and dimly lighted. Even the pews were showing significant wear and tear. They were cracked and falling apart, with kneelers that were heavy, awkward and, in some cases, broken, said Leon Poché, a member of the shrine’s board.

Just replacing the slate roof — a necessity to preserving the shrine — seemed out of reach financially. The prioress, grasping for ideas, was thinking of getting donated slates and volunteer labor. “It was a terrible idea, it would have never worked, but it showed how hopeless we were,’’ Dolce said.

But the Ursuline nuns, who look to Our Lady of Prompt Succor as a provider of speedy answers to prayer, experienced exactly that.

Phyllis Taylor, whose philanthropist husband Patrick Taylor had replaced a termite-riddled floor in the shrine 20 years earlier, heard about their leaking roof. She donated the money for a new one, which was installed in 2013.

“The thing about Phyllis Taylor was not only the money and that we replaced the roof. She gave us hope that we could do it,’’ Dolce said. “She was sent by Our Lady of Prompt Succor. She was the answer to our prayers.’’

While the roof was the most critical renovation, the nuns didn’t want to stop there. The entrance to the shrine wasn’t safe for worshippers, especially during the fall and winter when it’s dark at the end of the 5 p.m. weekday Mass.

The sisters also were thinking about the future needs of the aging building and wanted to create a $500,000 endowment to ensure its continued good repair. Their goal was to raise $2.5 million and finish the work in time for the 200th anniversary — a tall order, but one they expect to realize.

“Once Phyllis gave us the money, it just seemed like the money started coming in,’’ Poché said.

The nuns suddenly could afford to get building’s exterior waterproofed, the interior painted and the pews in the main chapel replaced. They also got new LED lighting and a modern sound system.

“That’s essential to worship in every church,’’ Dolce said. “The faithful need to hear the word of God.’’

Poché, who is one of 40 to 50 people who attend Mass daily at the shrine, said much remains to be finished in the weeks between now and the anniversary.

The main chapel is still empty, awaiting the arrival of the new pews, which have been built to look like the ones they are replacing. Scaffolding obscures the chancel, where the final painting work is underway.

Outside, the new entrance is still cordoned off, and worshippers must enter and exit through a side door until the $500,000 new entry is completed.

On Friday morning, as workers moved to and fro inside the shrine, some of the tile had buckled up, prompting new concerns. An inspection by a structural engineer determined it was caused by weather changes.

For Dolce, the work that is nearing completion is only the first step toward an expansion of the shrine’s outreach work.

“We would like to survey our faithful and ask what do they want the shrine to offer for their spiritual growth,” Dolce said, crediting Poché with the vision to do so.

“That’s what we’re going after, to have a place for people to come and have the shrine available,’’ Poché said.

The community of the shrine’s faithful includes locals like Poché and his wife, an Ursuline Academy alumna, but also people from all over the country. A group is coming this month from Houston to clean the pews in the small chapel so that they’ll be in tiptop condition for the anniversary. Another group from Springfield, Illinois, comes to New Orleans every Thanksgiving for a cruise but makes the shrine their first stop.

“These are all little things,’’ Dolce said. But they are signs that the shrine is, in her words, “a shrine of the faithful and for the faithful, and people feel they have ownership.’’

The hope of the Ursulines is that, with the building’s physical condition no longer a pressing concern, they can concentrate on making the shrine a place to nurture the faithful’s spiritual growth.

“The sisters all realize that in 10 or 15 years, there’s not going to be a lot of Ursuline nuns around, unfortunately,’’ Poché said. “It’s going to be up to the people to keep the shrine going. And what the nuns did not want to do was give the shrine to the people in bad condition.’’

For Dolce, the upcoming bicentennial has historic significance. Without the victory over the British invaders, the history of the nation would have been far different, she said. But it’s also a reminder of the power of faith.

“In spite of an impossible challenge that we should not have hoped to meet, we knew there was a mother, should we invoke her, who would come to our aid,’’ she said of the prayers of those long-ago Ursulines.

The lesson for today, Poché said, is that people should “renew their faith in the power of prayer.’’

Dolce readily talks about the miracles attributed to Our Lady of Prompt Succor, but the shrine isn’t just about miracles, she said. It is also about community.

“If the physical condition of the shrine is not consuming us, our focus is on the faithful, so that the community of the faithful will keep alive the belief in Our Lady of Prompt Succor as a source of hope and consolation,’’ she said.

Follow Sara Pagones on Twitter, @spagonesadvocat.