Retired Pope Benedict XVI has broken his silence to reaffirm the value of priestly celibacy, co-authoring a bombshell book at the precise moment that Pope Francis is weighing whether to allow married men to be ordained to address the Catholic priest shortage.

Benedict wrote the book, From the Depths of Our Hearts: Priesthood, Celibacy and the Crisis of the Catholic Church, along with his fellow conservative, Guinean Cardinal Robert Sarah, who heads the Vatican’s liturgy office and has been a quiet critic of Francis.

The French daily Le Figaro published excerpts of the book late Sunday; The Associated Press obtained galleys of the English edition, which is being published by Ignatius Press.

Benedict’s reaffirmation of priestly celibacy, however, gets to the heart of a fraught policy issue that Francis is expected to weigh in on, and could well be considered a public attempt by the former pope to sway the thinking of the current one.

The implications for such an intervention are grave, and are likely to fuel renewed anxiety about the unprecedented situation of two popes, one retired and one reigning.