Pope Benedict XVI walks down steps in St. Peter's Basilica at the Vatican Feb. 10, 2013, two days before he announced his resignation. (CNS photo/Paul Haring)

Vatican City — Retired Pope Benedict XVI has coauthored a new book defending the Catholic Church's practice of a celibate priesthood, in a shocking move that comes as Pope Francis is considering the possibility of allowing older, married men to be ordained as priests in the Amazon region.

According to excerpts from the volume released Jan. 12 by the conservative French outlet Le Figaro, the ex-pontiff says he could not remain silent on the issue as Francis is contemplating the move, which was requested by the bishops from the nine-nation Amazon region at October's Vatican synod gathering.

The book is co-written with Cardinal Robert Sarah, the head of the Vatican's liturgy office. It is to be released in France Jan. 15 and carries the title Des profondeurs de nos cœurs ("From the Depths of Our Hearts).

Although the volume is yet to be seen in full, it appears to signify something as yet unexperienced in the two millennia history of the Catholic Church: a retired pope openly weighing in on something currently under consideration by his successor, the reigning pontiff.

One noted theologian reached shortly after the release of the excerpts called Benedict's decision to write on the issue a "serious breach."

"It interferes with a synodal process that is still unfolding after the Amazon synod … and threatens to limit the freedom of the one pope," said Massimo Faggioli, a historian and theologian at Villanova University.