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Pope Emeritus Benedict has issued an ardent defense of clerical celibacy, breaking his pledged silence on major church affairs just as Pope Francis is considering an exception to 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 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 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 shortage of priests.

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 in a 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'.

Benedict and Francis have spoken admiringly of each other, but their different views — 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, Catholic program director at the 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 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.