The move was virtually unprecedented, as every Church head since Gregory XII in the early 15th century had fulfilled his papal duties until death. Furthermore, it led to the unique situation that plays out today, with one pope (Francis) and a pope emeritus (Benedict) both gracing the grounds of the Vatican in distinct white papal vestments.
Benedict cited his advanced age and deteriorating strength as the impetus for his resignation, explaining that his condition had forced him to "recognize my incapacity to adequately fulfill the ministry entrusted to me."
However, given the events that unfolded during his tenure, which exposed the Church's difficulties in adapting to a changing world and threatened the infallible status of its leader, the non-believers could be forgiven for assuming there were other factors that drove his decision.
Benedict inherited the fallout of the sexual abuse scandal that was gaining steam
Upon ascending to the papacy in April 2005, Benedict was forced to publicly confront the trauma of sexual abuse by Church clergy that had bubbled to the surface after generations of suppression.
The pope was already familiar with many of the troubling details, dating back to his days as prefect of the Congregation of the Doctrine of the Faith under John Paul II, and as such he was well suited to lead the Church into the 21st century on this issue.
Benedict ended the service of Father Marcial Maciel Degollado, an influential Mexican priest with a long trail of accusations, and he became the first pope to meet with sexual abuse victims in 2008. Two years later, he personally apologized to more victims in a pastoral letter to Ireland.
But Benedict also became ensnared in the wide-reaching controversy that year, when he was accused of transferring a known pedophile priest during his time as archbishop of Munich in 1980 (the Vatican said that a deputy was responsible for the transfer).
According to The New Yorker, the Church defrocked 384 offending priests from 2011-12, but this was a crisis that had outgrown the efforts of one pope. Underscoring the depth of the problem, the wrenching 2012 documentary Mea Maxima Culpa: Silence in the House of God, about the abuse of four deaf boys in the 1960s and the Church's attempts to bury the allegations, aired shortly before Benedict stepped down.
PHOTOS: Pope Benedict's Retirement
Italian authorities were watching the Vatican's finances
Another issue that proved problematic for the pope was the financial entanglements of the Vatican Bank.
Again, this was something that predated Benedict's papacy, as the Vatican Bank had long permitted the use of secretive accounts that invited scrutiny for potential money laundering. However, Benedict was drawn into the fray in 2010 when Italian authorities seized nearly $30 million of Vatican Bank funds that had been earmarked for transfer, with no satisfactory answer as to what the money was for.
Benedict sought to update the Church's archaic practices by establishing the independent Financial Intelligence Authority as a watchdog, but the entrenched bureaucracy limited the potential of effective change, and in May 2012, the Vatican Bank's president was fired for negligence.
The lack of transparency led to another embarrassing turn of events just before Benedict's exit, with the Vatican briefly relegated to a cash-only operation in early 2013 after Italian banks ceased doing business with the Holy See.
His butler leaked the pope's personal correspondence to a journalist
The straw that broke the camel's back may well have been the disclosure of the pope's personal documents in the "Vatileaks" scandal of 2012.
That year, in an investigative TV series and best-selling book His Holiness: The Secret Papers of Benedict XVI, Italian journalist Gianluigi Nuzzi revealed to the world a Church management divided by cliques and infighting, the withholding of important financial information from the pope, and the machinations behind the transfer of an archbishop who tried to enforce Benedict's reforms and rat out the uncooperative members of the flock.
Nuzzi's source was soon discovered to be the pope's butler, Paolo Gabriele, who admitted to exposing the private correspondence out of fear that corruption was overtaking the Church. Sentenced to 18 months in prison by a Vatican court, Gabriele was pardoned by the end of 2018 by Benedict, though the damage was already done.
Benedict maintains that he resigned for the right reasons
In the weeks that followed Benedict's retreat into private life, theories abounded as to why one of the world's most powerful spiritual leaders undertook the legacy-altering move. One conspiracy theory held that he was forced into resignation, but his ongoing use of his papal name and garments were signaling that he still considered himself the rightful head of the Church.
But Benedict has stuck to his original explanation, with a 2018 documentary, Benedict XVI: in Honor of Truth, providing supporting evidence that the pope emeritus felt he could no longer publicly lead the 1.2 billion Catholics who, more than ever, needed steadfast stewardship during times of immense change.
Meanwhile, he lives out his days writing and praying in the solitude of the Vatican's Mater Ecclesiae monastery, the full extent of the circumstances and struggles that drove him from the apex of his calling likely shared only with his Savior.