The pope emeritus rejected the idea that priestly celibacy is based on a contempt for human sexuality within the Church. He noted that this claim was also dismissed by the Church Fathers, and that the Church has always viewed marriage as a gift from God.
"However, the married state involves a man in his totality, and since serving the Lord likewise requires the total gift of a man, it does not seem possible to carry on the two vocations simultaneously," he said. "Thus, the ability to renounce marriage so as to place oneself totally at the Lord's disposition became a criterion for priestly ministry."
Just as the priests from the Tribe of Levi renounced ownership of land, priests in the New Covenant renounce marriage and family, as a sign of their radical commitment to God, he said.
This is seen in the Psalm prayed when a man entered the clergy before the Second Vatican Council, he said: "The LORD is my chosen portion and my cup; you hold my lot. The lines have fallen for me in pleasant places; yes, I have a goodly heritage."
Benedict's theological reflection is followed in the book by a set of pastoral considerations from Sarah.
"My bishop's heart is worried. I have met with many priests who are disoriented, disturbed and wounded in the very depths of their spiritual life by the violent challenges to the Church's doctrine," Sarah said.
"I speak up so that everywhere in the Church, in a spirit of true synodality, a calm, prayerful reflection on the spiritual reality of the sacrament of Holy Orders can commence and be renewed."
The cardinal called priestly celibacy "the expression of the intention to place oneself at the disposal of the Lord and of men and women," adding that "Priestly celibacy, far from being merely an ascetical discipline, is necessary to the identity of the Church."
Ordaining married men would create a "pastoral catastrophe," risking the Church's understanding of both the priesthood and itself, Sarah warned. "If we reduce priestly celibacy to a question of discipline, of adaptation to customs and cultures, we isolate the priesthood from its foundation."
"This total delivering of himself in Christ is the condition for a total gift of self to all men and women," he said. "He who has not given himself totally to God is not given perfectly to his brethren."
While some exceptions exist – such as when some married Protestant pastors become Catholic and are able to be ordained priests – the shortage of priests in isolated areas is not such an exception, he said. Ordaining married men in these communities "would prevent them from giving rise to priestly vocations of celibate priests," which would create "a permanent state detrimental to the correct understanding of the priesthood."
Sarah questioned whether the call for married priests among "isolated, poorly evangelized populations" is intended "to prevent them from discovering the fullness of the Christian priesthood."
The cardinal said that he has met with isolated communities who were living the faith through prayer and scripture without the support of priests and sacraments, similar to the situation faced by some communities in the Amazon. He recalled their "unimaginable joy" at being able to participate in a celebration of the Mass.
"Allow me to state forcefully and with certainty: I think that if they had ordained married men in each village, they would have extinguished the Eucharistic hunger of the faithful," he said. Ordaining married men would prevent young Churches from the experience of seeing themselves as the Bride of Christ and encountering Christ as Bridegroom through the radical encounter of a celibate priest, he said.
Sarah added that he would not be a priest today if it were not for his encounter with celibate missionary priests in his youth, because it was the radical nature of their lives that impacted him.
The cardinal also argued that "[t]he Eastern married clergy is in crisis," pointing to comments by some members of these Churches noting tension between the priestly and married states, as well as the problem of divorce by priests.
He also rejected calls for female ordination, while encouraging a deeper study of "the feminine charism," in order to properly recognize the role and contributions of women in the Church.
Concluding their work, Benedict and Sarah encouraged the discussion surrounding celibacy in the Church to be carried out with a proper understanding of the nature of the priesthood.
"It is urgent and necessary for everyone-bishops, priests and lay people-to stop letting themselves be intimidated by the wrong-headed pleas, the theatrical productions, the diabolical lies and the fashionable errors that try to put down priestly celibacy," they said.
They called for priestly celibacy to be examined through a "fresh look with the eyes of faith."
"This fresh look will be the best rampart against the spirit of division, against the spirit of politics but also against the spirit of indifference and relativism," they said.
On Jan. 13, Andrea Tornielli, editorial director for the Vatican's Dicastery for Communications, wrote an op-ed praising the book.
"Ratzinger and Sarah - who describe themselves as two Bishops 'in filial obedience to Pope Francis' who 'are seeking the truth' in 'a spirit of love for the unity of the Church' - defend the discipline of celibacy and put forth the reasons that they feel counsel against changing it," Tornielli wrote.
This story is developing and will be updated.