Pope Benedict XVI's most lasting legacy may be as a teacher, theologian and writer of more than 60 books over a half century.
But he also will go down in history as the first pope in centuries to accept that the job is too much for a frail old man.
"It shows his humility," said the Rev. D. Vincent Twomey, who has known the pope for more than 40 years and wrote the 2007 book Pope Benedict XVI: The Conscience of Our Age.
"He didn't see himself as necessary for the church," said Twomey, a former student of Joseph Ratzinger who would become Pope Benedict XVI at the University of Regensburg in Germany.
Twomey last saw his old teacher in August, when the pope had his yearly meeting with former students. Twomey wondered at the time how much longer Benedict could maintain his pace, since his health was clearly fading.
Religion journalist David Gibson said it was inevitable that a pope someday would confront the topic of retirement, since modern medicine allows popes to live longer lives and modern media require that they do so publicly.
"It's a younger man's job," Gibson said, "and the popes know this and know they have to adapt to the modern world."
Although Benedict signaled in 2010 that he would resign if he felt it necessary, Gibson was "shocked" by the news. In his 2007 book, The Rule of Benedict: Pope Benedict XVI and His Battle With the Modern World, Gibson wrote that popes "do not retire."
His decision was courageous, Gibson said, and in keeping with much of Benedict's pontificate, in which he overcame, to some extent, detractors' characterizations as "God's Rottweiler" and "Panzerkardinal."
"He wanted to put the focus on Jesus Christ, not on the pope," Gibson said. "That's what he did with the papacy and that's what he did by resigning."
Benedict's most recent book as pope, in fact, was the third in the "Jesus of Nazareth," series, titled The Infancy Narratives.
Twomey and another former student, the Rev. Joseph Fessio, editor and chief of San Francisco-based Ignatius Press, said those characterizations were false in the first place.
Ratzinger never wanted to be a bishop, cardinal or pope. But as prefect for the Congregation for the Doctrine of the Faith, a job he held from 1981 until becoming pope in 2005, it was his job to correct errors and abuses in the church.
"People in error don't like to be told that," Fessio said. Ignatius Press has published the English editions of more than 50 of Benedict's books.
"He was always a sign of contradiction," Twomey said. "The values [he holds] and truths are at variance with the values of the contemporary world."
Fessio believes one of Benedict's biggest and little-known legacies will be the bishops he has appointed. "He has done a masterful job around the world," Fessio said, "and we see a whole cadre of strong, intelligent and loyal bishops now."
John Hunt, a Utah Valley University history professor whose specialty is the Renaissance papacy, believes Benedict's legacy will be as a transitional pope the last of those to have lived during World War II and perhaps the last European before Catholics regard an African or Latin American as the Vicar of Christ on Earth.
The Rev. Joseph W. Koterski, who teaches philosophy at Fordham University, said Benedict's legacy will be "that he sustained John Paul II's vision that the Second Vatican Council is in continuity with the entire history of the church."