In a letter to Italian mathematician Piergiorgio Odifreddi, who is an atheist, the former Pope, Benedict XVI, affirmed the professor's desire for open and honest dialogue as the path to truth.

"Frankness is a part of dialogue; only in this way can knowledge increase," the emeritus Pope reflected in his 11-page letter, portions of which were published Sept. 24 in Italian daily "la Repubblica."

Odifreddi received the letter Sept. 3, and publicized portions of it with Benedict's permission. He explained that the letter is an engagement with his 2011 work, "Dear Pope, I'm writing to you." The book, which is an introduction to atheism and a refutation of Benedict's 1968 work "Introduction to Christianity," was given to Benedict by a mutual friend of Odifreddi and Archbishop Georg Ganswein.

"But that he could respond, and also comment on it profoundly, was beyond reasonable hope," Odifreddi said in his blog post introducing Benedict's letter. He described receiving the former Pope's letter as surprising and quite emotional for him, particularly since "atheism regards reason, while personalities and symbols of power act on feelings."

He described Benedict XVI as having taken seriously his arguments and not ignoring them, for which he is thankful. He read Benedict's "Introduction to Christianity" and realized that his "faith and teaching … in contrast to those of others, were sufficiently cohesive" to engage with serious dialogue.

Odifreddi said his book was "obviously, not meant 'to convert the Pope,' but to honestly explain the perplexities" he has regarding faith. He noted that the result has been "a dialogue between faith and reason" that has "allowed both to frankly confront" the other, as Benedict had called for in his 2009 Christmas address to the Roman Curia, when he called for dialogue with "those to whom religion is something foreign … who nevertheless do not want to be left merely Godless, but rather to draw near to him, albeit as the Unknown."

The atheist mathematician noted that while he and Benedict are "divided on nearly everything," they are "united at least on one objective: the search for Truth, with a capital T."

Benedict began his letter by thanking Odifreddi for his sincere and just engagement with "Introduction to Christianity" and with his faith, which he said was "precisely and in great part what I had intended" by his 2009 Christmas address to his curia.

The emeritus Pope's own frankness was displayed by his "mixed" opinion of Odifreddi's book, which he called at points enjoyable and profitable, but also at times "aggressive" and "reckless."

Benedict took issue with Odifreddi's characterization of theology as "science fiction." He defended theology as a field of genuine knowledge and truth by noting that different fields have different criteria of knowledge, and that mathematics cannot be the standard for other sciences; that theology has produced lasting results; and that science itself has had its own strains of "science fiction."

He also pointed out that an important function of theology is "to maintain a link between religion and reason" and vice versa. "Both functions are of essential importance for humanity," he noted, because "there exist pathologies of religion and – no less perilous – pathologies of reason. Each needs the other, and to keep them continually connected is an importance competence of theology."

Benedict also took issue with Odifreddi's "ostentatious" presentation of clerical sex abuse as "a filth peculiar to Catholicism." Agreeing that the scandal is of "deep concern" and that it is a "suffering … that the power of evil should penetrate to such an extent in the inner world of faith," he pointed out that such abuse is not specific to priests.

He added that while there are gravely immoral acts committed by Church members, the "great shining path of goodness and purity" also found in the Church cannot be forgotten. He pointed out the examples of Saints Benedict, Francis and Clare of Assisi, Teresa of Avila, John of the Cross, Vincent de Paul, and Mother Teresa, saying that "it is true today also that the faith leads many persons to selfless love, at the service of others, to sincerity and to justice."

Next, Benedict defended the historicity of Jesus Christ, and added that while the historical-critical method of exegesis can be abused, as was stressed by 19th century Russian theologian Vladimir Soloviev, it is "necessary for a faith which does not propose myths by using historical images, but calls for a true history and therefore must present the historical reality of its claims in a scientific way."

The former Pope then found more common ground with Odifreddi in their treatment of the prologue to John's Gospel – while still finding shortcomings in the atheist's interpretations.

"In your religion of mathematics," Benedict wrote, "three fundamental themes of human existence are not considered: freedom, love, and evil."

"Whatever neurobiology may say or not say about freedom, in the real drama of our history it is present as a determining reality and must be taken into consideration. Yet your mathematical religion knows nothing about evil."

"A religion that ignores these fundamental questions remains hollow."

Benedict concluded by saying his criticism of Odifreddi's book was "tough in part," but went on to say that only this frank attitude to dialogue can lead to a mutual discovery of Truth.

"In any case, however, I value very highly the fact that you, through your engagement with my 'Introduction to Christianity', have sought such an open dialogue with the faith of the Catholic Church and that, notwithstanding all the contrasts, at the central part, there is no lack of convergences."