Only her deep faith.  There are no words that can possibly state how much heartache there is in my heart when I read about this young woman.

American hostage Kayla Mueller was tortured, verbally abused, forced into slave labor for ISIS commanders in Syria and raped by the group’s top leader, but her fellow hostages say she never surrendered hope, she selflessly put the welfare of fellow captives above her own and she even stood up to executioner “Jihadi John” to defend her Christian faith.

Four former hostages who shared cells with Mueller, speaking publicly for the first time about their shared ordeal for ABC News’ “20/20” broadcast, “The Girl Left Behind,” airing Friday, say the Prescott, Arizona, humanitarian aid worker was a courageous 25-year-old who inspired them.

Their ISIS guards were overseen by the British tough Mohammed Emwazi, who would later be dubbed Jihadi John, as he carried out the beheadings and killings of 10 hostages. The Londoner led three other Britons who oversaw the hostage operation. Their prisoners called them “The Beatles.”

In March 2014, Mueller was taken to a room next door several times where male hostages were being held. Former hostages said Emwazi paraded her in front of them to show prisoners about to be released who she was and to offer her own proof-of-life by removing her head scarf and briefly introducing herself.

Former hostage Daniel Rye Ottosen, a Danish freelance photographer, recalled how Mueller turned the tables on the men in black.

“One of the Beatles started to say, ‘Oh, this is Kayla, and she has been held all by herself. And she is much stronger than you guys. And she’s much smarter. She converted to Islam.’ And then she was like, ‘No, I didn’t,'” Ottosen told ABC News.

He admits it surprised him a lot. He had once tried to strangle himself when ISIS guards strung his arms up by chains.

“I would not have had the guts to say that. I don’t think so,” he said. “It was very clear that all of us were impressed by the strength that she showed in front of us. That was very clear.”

The only period in Mueller’s 18 horrifying months as an ISIS hostage when she wasn’t subjected to some form of torture, verbal abuse, prolonged isolation, sensory deprivation, stress positions, forced labor or sexual assault before she died in captivity was the six weeks she was held at an abandoned oil refinery in Syria, with other Western hostages in 2014.

Because many of the ISIS captives were being individually negotiated for ransom with their governments, employers or families, the women, at least, inside the makeshift prison south of ISIS stronghold Raqqa were not subjected to the kind of abuses that Mueller said she experienced in other ISIS prisons before and after her time there, according to the Mueller family and those held captive with her.

Three of the Westerners released by ISIS and a Yazidi teenager who escaped captivity provided eyewitness accounts to ABC News of Mueller’s strength, selflessness and will to survive amid her considerable suffering, including details she gave them of her treatment when she was completely alone for most of her confinement by the terrorist group.

“They would scream at her, and they would, you know, blame her for everything that America has done in the world,” Frida Saide, one of three women from Doctors Without Borders who shared a cell with Mueller at the oil refinery, told ABC News in an interview this month.

“They picked her apart,” said Patricia Chavez, one of the other Doctors Without Borders aid workers held with Mueller.

In her seventh month of captivity, Mueller’s frequent isolation and moves between makeshift prisons in Aleppo and Raqqa was interrupted by the arrival at the oil refinery of Saide, Chavez and a dozen other hostages, including Europeans in the process of being ransomed.

In March and April 2014, the women from Doctors Without Borders and a French journalist carried out three letters Mueller wrote by hand to friends and family, indicating it was finally her turn. The Doctors Without Borders women were made to memorize an ISIS email address, which the hostage takers instructed them to give to her parents.

That eventually led to extraordinary negotiations for her release, the former hostages and the Mueller family said in an ABC News investigation spanning more than two years.

Saide, 35, from Sweden and Chavez, 35, from Peru and Belgium, had not been publicly identified as ISIS hostages before agreeing to speak to ABC News this month about their friend, Mueller. At least six men held with them were eventually executed by the brutal “Beatles,” and the experience has left the women traumatized.

“Fear. It’s fear of the unknown. You don’t know what’s going to happen,” Chavez recalled of the state of terror in which they lived.

Rest in peace.