This combination of sex scandals and priestly shortfalls confronts traditionalists with the challenge of defending celibacy’s relevance anew. In “From the Depths of Our Hearts,” to be published next month, former Pope Benedict XVI and Cardinal Robert Sarah of Guinea each give an explanation grounded in theology, not sociology.

Benedict considers celibacy a main link binding the new (Catholic) covenant to the old (Jewish) one. It is an outgrowth of Old Testament ideas of priestly abstinence during sacrifices, extended to reflect the daily responsibilities of a Christian priest. He notes that the Eucharistic prayer draws on Deuteronomy for its language of what it means to “stand before God and serve him.” The Christian priest’s renunciation of his share in God’s physical creation has a parallel in God’s promise to Israel — all Israel’s tribes had right to land except the priestly tribe of Levi. Levites therefore had to live “by God and for God,” just as the unwed priest does.

Neither Benedict nor Cardinal Sarah is naïve or prudish. Celibacy is not asexuality. It is sexuality regulated in a special way. Its logic rests on an elaborate analogy between holy orders and holy matrimony. The church is the bride of Christ. Just as a married man must give the whole of himself to his wife, a celibate priest must give the whole of himself to the church.

Still, the non-Catholic reader who does not follow Cardinal Sarah and Benedict’s reasoning closely and patiently is likely to find it weird. Cardinal Sarah writes, “Every time a priest repeats ‘this is my Body,’ he offers his body, as a man” — “son corps sexué,” in the French original — “in continuity with the sacrifice on the Cross.” This is the sense in which celibacy is a “gift to the church,” a formula of the reformist Pope Paul VI that many priests, including Pope Francis himself, are fond of repeating. Note well, though — it is a gift to the church, not to the individual priest’s character-building or his emotional needs.

This complicates the worldly problem that roiled Pope Francis’s recent Amazon deliberations. Whatever its theological justification, is celibacy shutting out otherwise eligible young men who would make good priests? That is the view of Erwin Kräutler, the Austrian-Brazilian bishop who for 35 years headed the Amazonian prelature of Xingu. He claims the natives there do not even understand celibacy and pity those who practice it. “When I go to an indigenous village,” Bishop Kräutler said in October, “the first thing they ask is: ‘Where is your wife?’”

Others, however, including Cardinal Sarah, believe this is an “error in perspective.” Cardinal Sarah is an important bridge figure. He is conservative and he has been a priest for 50 years. But missionaries brought his Guinean parents into the church, and he grew up among non-Christians. “The younger a Church is, the more she needs an encounter with the radical character of the Gospel,” he explains, insisting that if his village had been evangelized by a married man, he would not today be a priest. “The radical character of the missionaries’ life is what attracted me.”

On both sides of this debate, everything seems to be tied up with celibacy. For Cardinal Sarah, as long as the traditional conception of celibacy holds up — the conception that a priest celebrating Mass “offers his body, as a man” — there is an ironclad logic against ordaining female priests. Should celibacy be rethought, he admits, the logic of female exclusion would grow weaker, as would the traditional conception of the priesthood and, with it, the traditional conception of the church.