Now 88, Pope Benedict has met annually since the 1970s with what is known as the "Ratzinger Schulerkreis" (Ratzinger Student Circle), which is made up of bishops and scholars who earned their doctorates under him in Germany.

The schulerkreis gathers for a week of theological discussions; the topic this year was "How to speak about God today" and was by Msgr. Tomas Halik, a Czech theologian and winner of the 2014 Templeton Prize.

The retired pope did not join his former students for the discussions in Castel Gandolfo, but spent the morning with them Aug. 30 in the Vatican's Teutonic College where the Joseph Ratzinger-Benedict XVI Roman Library will open to scholars in November.

In the day's Gospel reading from St. Mark, Jesus says, "Nothing that enters one from outside can defile that person; but the things that come out from within are what defile."

According to a post on the Ratzinger Foundation website, Pope Benedict said he remembered that when the same Gospel was read three years ago at Mass with the schulerkreis, Austrian Cardinal Christoph Schonborn of Vienna, one of the retired pope's former students, had asked whether perhaps it is true that people also must take measures to purify themselves or protect themselves from what comes from outside.

The retired pope said the answer, found looking at the entire Gospel, would indicate that people must take precautions to avoid "the many illnesses, even epidemics, which threaten us."

However, "exterior hygiene" is not enough, he said, because it is "the epidemic of the heart" that leads to corruption and other attitudes "that make people think only of themselves and not of what is good."

"What makes a person pure? What is the authentic source of purification? How does one achieve the hygiene of the heart," Pope Benedict asked.

Jesus himself told the disciples that his word would make them pure, he said. "The Word is much more than words because it is through words that we encounter the Word, Jesus himself. And we encounter the Word also in those who reflect it, who show us the face of God and reflect his meekness, his humility of heart, his simplicity, his love, his sincerity."