ATHENS — Pope Emeritus Benedict XVI has issued an ardent defense of clerical celibacy, breaking his pledged silence on major church affairs just as Pope Francis is considering an exception that would allow some married men to serve as priests.

Benedict’s remarks, revealed in a new book excerpt published Sunday by the French newspaper Le Figaro, cast light on a once-unthinkable dynamic inside the Roman Catholic Church: A former pope trying to influence his successor in whether the church heeds or breaks with its traditions.

“The ability to renounce marriage in order to place oneself fully at the disposal of the Lord has become a criterion for priestly ministry,” Benedict XVI writes in the book he has co-authored.

In the excerpts, Benedict invokes his own ordination and calls celibacy a sometimes “painful” but necessary step. Though Francis has also defended celibacy — calling it a “gift” to the church and saying it should not be optional — some of the Argentine pontiff’s allies have pushed for exceptions, saying the priesthood needs to modernize and find ways to make up for a severe shortage of vocations.

Bishops meeting in Rome last year recommended that Francis allow the ordination of married men in the particularly remote Amazon region, an endorsement that some traditionalists warned might set off a broader weakening of the church’s millennium-old celibacy requirement. Francis is considering whether to affirm the recommendation.

But no matter what Francis decides, Benedict’s willingness to speak out risks the kind of inner-church tension that analysts worried about when he abdicated seven years ago.

After he stepped down, Benedict — who lives inside a Vatican monastery — vowed silence on key issues to give room for Francis. But he has twice broken that vow in less than a year, with the excerpt Sunday and the release in April of a lengthy letter devoted to clerical sexual abuse in which his theories often contradicted Francis’s.

Benedict and Francis have spoken admiringly of each other, but their different views about the church — Francis has pushed for changes that his predecessor opposed — have caused some traditionalists to rally around Benedict as an alternative authority figure.

“One Pope is complicated enough,” John Gehring, the Catholic program director at the Washington-based advocacy group Faith in Public Life, wrote on Twitter Sunday night. “This is a mess. With great respect to Benedict XVI, it’s time for him to live up to his promise to be ‘hidden from the world.’ ”

“From the Depths of Our Hearts,” was co-written by Benedict and the Guinean Cardinal Robert Sarah, with each authoring certain passages. Sarah, who did not immediately respond to a request for comment, is far more direct than Benedict, speaking to Francis directly about the dangers of altering the church’s celibacy practices.

“I am humbly pleading for Pope Francis to protect us definitively of such an eventuality by putting his veto to any weakening or lessening of priestly celibacy, even limited to one region or the other,” wrote Sarah, the head of the Vatican’s liturgical office. “The possibility to ordain married men would represent a pastoral catastrophe, an ecclesiastical confusion and an obfuscation in an understanding of the priesthood.”

A passage jointly written by Sarah and Benedict mentions that they had taken note of the “uproar” surrounding the bishops’ meeting on the Amazon last year. Benedict and Sarah wrote that they could not stay silent.

“If ideology divides, truth unites hearts,” they wrote. “Examining the doctrine of salvation can only unite the Church around its divine Master. We do it in a spirit of charity.”

Benedict, 92, uses a walker and talks barely above a whisper, according to recent footage, but remains mentally sharp. His contributions to the book, according to the excerpts, are steeped in church language. He makes the case that celibacy is a way for priests to give themselves fully to the service of the priesthood.

“To be with God is to set aside what is only the self,” Benedict writes.

Benedict’s personal secretary, Archbishop Georg Gänswein, did not respond to a request for comment.

As the church debated last year whether to allow the ordination of married men in the Amazon, traditionalists warned about the destruction of the priesthood. There are already some celibacy exceptions within the church: married Anglican ministers, in some cases, can join the Catholic priesthood after conversion. But some conservatives worry that the rationale for the Amazon could also be applied to other parts of the world, including Europe and North America, that have shortages of priests.

Sarah argues that lifting the celibacy requirement would not help such areas, but deprive them of true priests.

“We cannot offer them ‘second class’ priests,” Sarah wrote.

Quentin Ariès in Brussels and Stefano Pitrelli in Rome contributed to this report.