There are endless meetings at the Vatican with clergy, diplomats and heads of state. This year he completed hour-long meetings with every bishop in the United States, according to Bishop Thomas Paprocki of Springfield, Illinois.

"It's a grueling and demanding schedule to keep up with," Paprocki said.

As the pope, "there's an expectation you're going to be doing trans-Atlantic flights and his doctors have warned him against it the whole time," Winters said.

Last year alone the pope traveled to Mexico, Cuba, and Lebanon.

While the most plausible explanation for his resignation seems to be the most benign, there are other elements of scandal and mismanagement at the Vatican that may have also played a role.

"No one is going to say this was a well-managed papacy," Winters said.

There were scandals that rocked both the church as a whole and the tightly knit community in Vatican City.

The child sex abuse scandal continued to plague the church globally even as strict reforms were put in place. A visible sign of the scandal at the coming conclave to select a new pope will be Cardinal Roger Mahoney, the former archbishop of Los Angeles, who was stripped of his public and administrative duties this month by his successor, Archbishop Jose Gomez, for his role in covering up a child sex abuse scandal. A spokesman for the Archdiocese of Los Angeles confirmed Mahoney will be attending the conclave.

Inside the walled compound of the Vatican City, the Vatican Bank is being investigated for noncompliance with European money laundering protections. The head of the bank left in disgrace.

The pope saw his own butler betray him by stealing documents from his desk and passing them to journalists, and internal battles erupt over alleged mismanagement.

On Sunday, the pope tweeted, "We must trust in the mighty power of God's mercy. We are all sinners, but His grace transforms us and makes us new."

The mention of personal sin was not out of character with the Christian belief outlined in Paul's letter to the Romans that "all have sinned and fall short of the glory of God."

In his statement Monday he again turned to flaws, saying, "Dear Brothers, I thank you most sincerely for all the love and work with which you have supported me in my ministry and I ask pardon for all my defects."

Lombardi, the Vatican spokesman, said he expects a new pope will be in place in time for Easter.

The pope gave little indication of what his future might hold, where he would live and what life for a former pope might entail. He concluded his statement by saying, "I wish to also devotedly serve the Holy Church of God in the future through a life dedicated to prayer."