VATICAN CITY (Reuters) - Former Pope Benedict, in a new book written with a conservative cardinal, defends priestly celibacy in the Catholic Church in what appears to be a strategically timed appeal to Pope Francis to not change the rules.

Benedict wrote the book, “From the Depths of Our Hearts,” with Cardinal Robert Sarah, 74, a Guinean prelate who heads the Vatican’s Congregation for Divine Worship and the Discipline of the Sacraments.

Excerpts were published on Sunday on the website of the French newspaper Le Figaro. The Vatican had no immediate comment on the book, which is due to be published on Monday.

In October, the final document of an assembly of Roman Catholic bishops, or synod, from the Amazon proposed that married men in the remote area be allowed to be ordained priests, which could lead to a landmark change in the Church’s centuries-old discipline of celibacy.

Pope Francis will consider it, along with many other proposals on issues that emerged during the synod, including the environment and the role of women, in a document of his own, known as an Apostolic Exhortation. It is expected to be issued in the next few months.

In 2013, when he became the first pope in 700 years to resign, Benedict, who lives in the Vatican and is now 92 and in failing health, vowed to remain “hidden from the world”.

But he has given interviews, written articles and contributed to books, in effect breaking that pledge and cheering conservatives, some of whom do not recognize Francis’ legitimacy.

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Massimo Faggioli, a theologian at Villanova University in the United States, called it “a serious breach” by the former pope, who vowed “unconditional reverence and obedience” to his successor.

In his part of the book, Benedict says celibacy, which became a stable tradition in the Church only about 1,000 years ago, carries “great significance” because it allows a priest to concentrate on his vocation. He says “it doesn’t seem possible to realize both vocations (priesthood and marriage) simultaneously.”

In a joint introduction, both men say they could not remain silent about the October synod, which at times led to clashes between progressive and conservative Catholic media outlets, underscoring the polarization in the 1.3 billion-member Church.

The proposal calls for older married men who are already deacons in the Church, have a stable family relationship and are proven leaders in their communities to be ordained as priests after adequate formation.

This solution to the shortage of priests, backed by many South American bishops, would allow Catholics in isolated areas to attend Mass and receive the sacraments more regularly.

For his part, Sarah says making exceptions to the celibacy rule would be a “a lie” that would set a dangerous precedent.