VATICAN CITY — Pope Benedict XVI's historic decision to resign at the end of February has astonished and perplexed the world in many ways, not least because of what might be called the mystery of the missing encyclical.

In December, the Vatican spokesman, Jesuit Fr. Federico Lombardi, said that Pope Benedict's fourth encyclical would be released in the first half of 2013.

Treating the subject of faith, the encyclical would complete a trilogy on the three "theological virtues," following "Deus Caritas Est" (2005) on charity, and "Spe Salvi" (2007) on hope.

Then, on the day after the pope's announcement, Fr. Lombardi announced that Pope Benedict would not issue another encyclical after all.

The news was surprising because it suggested that Pope Benedict, a former professor who has placed a priority on his teaching role as pope, had abandoned the most prominent teaching project of his pontificate just before its completion. This, even though Fr. Lombardi said that the pope had pondered resignation for several months, and the Vatican newspaper reported that he first considered the move in March 2012.

It was hardly plausible that so prolific an author might be suffering from writer's block, even given the deteriorating "strength of mind and body" he cited in announcing his resignation. Three days after that announcement, Pope Benedict delivered a highly structured, 46-minute long public talk, without a prepared text and only occasionally consulting his notes.

But unlike an off-the-cuff speech, papal encyclicals are not one-man productions. Though the pope ultimately determines their content, they are typically the fruit of much behind-the-scenes collaboration with Vatican officials and often with outside consultants as well. Pope Benedict's last encyclical, "Caritas in Veritate" (2009), appeared more than a year after its expected date, reportedly because of complications in this process. It is likely that such was the case again this time.

Fr. Lombardi has suggested that the former Pope Benedict might eventually publish the document under his own name, in which case it would not rank as part of the papal magisterium. But it is at least as likely that his successor will take up and finish the task.

Popes tend to honor their predecessors' commitments, which is why everyone assumes that the next pope will travel to Rio de Janeiro for World Youth Day in July. Indeed, Pope Benedict's own first encyclical, "Deus Caritas Est," was started by his predecessor, Blessed John Paul II.

If the next pope does finish the encyclical on faith, there is reason to think that his predecessor will be happy to have left it incomplete.

A major papal document whose production bridged the transition between the two pontificates could serve as a reassuring sign of continuity after Pope Benedict's practically unprecedented move. At the same time, since the next pope would undoubtedly stamp the encyclical with his distinctive priorities and style, it would exemplify Pope Benedict's ideal of reform as "innovation in continuity" with church tradition.

Pope Benedict has been careful throughout his pontificate to distinguish his personal writings from his papal documents, by publishing his bestselling series of "Jesus of Nazareth" books under the name Joseph Ratzinger. The knowledge that the next encyclical was the work of more than one pope would further underscore its impersonal character and reinforce the idea, which Pope Benedict has conveyed so dramatically through his resignation, that the papacy is an office distinct from any individual who might hold it.

Only three days before he announced he would step down, the outgoing pope said something that has acquired a more personal meaning in light of that historic event. Commenting on the First Letter of Peter to an audience of seminarians, Pope Benedict noted internal evidence that the apostle and first pope was not the epistle's sole author.

"He does not write alone, an isolated individual, he writes with the help of the church," Pope Benedict said. "Peter does not speak as an individual, he speaks 'ex persona Ecclesiae,' he speaks as a man of the church … He does not want to say only his word, but truly carries in himself the waters of the faith, the waters of all the church, and precisely this way gives fertility, gives fecundity and is a personal witness who opens himself to the Lord."