A mystical New Jersey nun who took vows on her deathbed will become the first person beatified on American soil — a historic moment that might not have happened but for a misplaced letter that languished between two file folders for a quarter-century.

It was a note from a grateful mom who was convinced that prayers to Sister Miriam Teresa had cured her young son of encroaching blindness years before, a medical mystery that would eventually become the first of two miracles needed for sainthood.

"That letter sat there in the filing cabinet for 27 years," said Dr. Mary Mazzarella, a retired pediatrician who was recruited by the local church to investigate the mother's claim before presenting the findings to the Vatican. "Just finding it was some kind of miracle."

In the Catholic Church, there are three major steps to getting a halo: the first is being venerated, or recognized for heroic virtue; beatification comes next, after the pope validates a miracle; a candidate can then be sainted through canonization after the declaration of a second miracle.

The entire process can take hundreds of years, and only a handful of Americans have been beatified or canonized. The ceremonies have traditionally taken place in Rome, and Miriam Teresa's Mass on Saturday at the Cathedral Basilica of the Sacred Heart in Newark will be the first of its kind in the United States.

A campaign to put Miriam Teresa Demjanovich on the road to sainthood was launched in 1945 by admirers of the Sisters of Charity nun.

A daughter of Slovakian immigrants who delayed entering the convent so she could care for her dying parents, she reported having religious visions: the Virgin Mary in an illuminated sky from her dorm window and Saint Therese of Lisieux walking across campus with her.

A college photo of Sister Miriam Teresa Demjanovich Courtesy Sisters of Charity of Saint Elizabeth

She was also recognized for spiritual writings she penned as a nun-in-training before a burst appendix killed her in 1927 at the age of 26, just weeks after she took the vows of poverty, chastity and obedience. According to Mazzarella, a nurse tending to her found a crucifix had mysteriously moved from a dresser to her chest.

Promoters of her cause began building a dossier and encouraging people to pray to her. One day in 1964, a fellow member of her order gave a religious memento to a third-grade student, Michael Mencer, who had just been diagnosed with juvenile macular degeneration.

The boy had been having trouble seeing for months. It began with walking into trees, dropping balls and wobbling on his bike and took a dire turn when he walked into a moving car. His mother, Barbara, brought him to an ophthalmologist, who gave the family a dreaded diagnosis.

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"The doctor told my mother that in six months, I'll be totally blind," Mencer, now 58, said from his home in Nebraska. "But I had this feeling that everything would be all right."

Michael Mencer, 58, stands for a portrait inside St. Joseph Catholic Church in Lincoln, Neb. Lauren Justice / for NBC News

The day his teacher gave him the memento — a prayer card and a strand of Miriam Teresa's hair encased in plastic — Mencer says he decided to walk home alone, even though he had only peripheral vision by then.

"I was about two blocks from the house when I think it happened," he said. "I looked up at what I thought was the sun, and it didn't hurt my eyes, but I could see an orb, a bright light. And when I looked back down I could see the hair in the memento," he said.

When he got home, he handed his mother the memento "and she could feel it was warm and she felt everything was going to be all right," said Sister Mary Canavan, who came out of retirement 18 months ago to shepherd Miriam Teresa to beatification.

Michael Mencer was eight years old when encroaching blindness suddenly reversed.

Weeks later, Mencer's family moved from Teaneck to south Jersey, and his mother took him to a top eye clinic in Philadelphia for a followup. Specialists delivered startling news: Michael had perfect vision.

Barbara Mencer credited Miriam Teresa with this reversal of fortune and said so in a letter she wrote in 1971 to a priest she learned was in charge of the sainthood mission.

"The letter was lost, caught between two files," Canavan explained.

Miriam Theresa's supporters continued trying to make her case. In 1979, she was exhumed from her grave in the Sisters of Charity cemetery so her remains could be examined. If the body was intact, it could be considered a marker of sainthood.

As it turned out, Miriam Teresa's corpse was not "uncorrupted," but she was moved from the grave where visitors — up to 1,000 a year — had been chipping off pieces of of a granite cross marker and was interred in a chapel crypt.

Testimonies about Miriam Teresa's life were still being collected and sent to Rome when her champions caught a break: A nun clearing out a file cabinet in 1998 found the long-lost note from Michael Mencer's mother.

Mazzarella, who had been appointed medical consultant for the cause, realized his case represented exactly the kind of cure needed under Vatican rules, which dictate a miracle must be spontaneous and permanent.

Sister Mary Canavan, of the Sisters of Charity of Saint Elizabeth, and Dr. Silvia Correale, a vatican representative working on the beatification of Sister Miriam Teresa, look at a book chronicling the paperwork involved in Sister Miriam Teresa's beatification. John O'Boyle / for NBC News

Although 27 years had passed since she wrote the letter, Barbara Mencer still had crucial documentation, including the eye doctor's written diagnosis and her application for services from the New Jersey Commission for the Blind.

But Mazzarella had more sleuthing to do. She tracked down a phone number for the doctor who examined Michael when he first starting having problems. He'd since died, but a woman at the office dug through his old records and found the file.

"This is 36 years later!" Mazzarella said. "If you don't think this is the hand of God who wanted this done...."

The doctor's notes confirmed he dilated the boy's eyes and found black pigment in the retina's macula, a sign of degeneration, she said. Four ophthalmologists who reviewed the evidence agreed the condition should have been incurable, she said.

The archdiocese of Newark set up a tribunal — "like a little trial," Mazzarella says — to evaluate the case. A six-member panel questioned Mencer's parents and three doctors, and their findings were translated into Italian, sealed in wax and sent off to Rome.

In 1964, Michael Mencer was told by doctors that he would be blind within months. He was given a piece of hair from Sister Miriam Teresa Demjanovich and once he received it, he says, he was able to see clearly. Lauren Justice / for NBC News

The paperwork sat there for eight years, because it wasn't until Pope Benedict announced Miriam Teresa's veneration in 2012 that the Vatican could look at the medical case. Seven more doctors reviewed the papers and agreed that the requirements for a miracle had been met; then the theologians were convinced the cure came through prayer.

On Dec. 13, 2013, Pope Francis declared that Miriam Teresa had performed a miracle, paving the way for Saturday's ritual, in which a cardinal from Rome will proclaim her beatified before a procession up the center aisle of the majestic cathedral.

Carrying the relic with a piece of the newly minted blessed's hair will be Michael Mencer. Fifty years after he was told to prepare for a life shrouded in darkness, he says his vision is still "pretty good" and he thinks often of the nun who died decades before he was born.

"It's like having a friend," he said. "And you know, you want to see your friends do well."