Q ­— I hear Pope Francis will write an encyclical on the environment. How do encyclicals differ from, say apostolic exhortations? What encyclicals from the past have impacted society the most, in your opinion?

A — In modern Catholic usage an encyclical is a letter written by the pope and circulated throughout the entire church. An “Apostolic Exhortation” is “a document issued by the pope in response to the deliberations of an episcopal synod” (following Richard R. Gaillardetz, By What Authority? page 80).The first pope to make systematic use of the encyclical letter as a favored form of teaching was Pope Benedict XIV (1740-1758). Encyclicals were to develop in both number and range.

The example comes to mind in the 19th century of Pope Leo XIII, who frequently used encyclicals as a method of teaching the faithful. Church historian Eamon Duffy writes: “One of the attractions of Leo’s encyclicals is that they rarely merely condemn, but we should not allow relief to blind us to the radical shift in the nature of papal teaching which his collected encyclicals represent. Here, for the first time, we have the Pope as an inexhaustible source of guidance and instruction. No pope before or since has come anywhere near his eighty-six encyclicals.” (Eamon Duffy, Saints and Sinners, page 317).

To decide what encyclicals have had the greatest impact either on the church or on society in general is beyond my competence to say with conviction. However, let me point to three that seem to me especially significant in this regard. First, Pope Pius XII, Divino Afflante Spiritu (1943), an encyclical that encouraged Catholic Scripture scholars to make use of all the modern techniques of biblical study in order to break open the Word of God for the faithful. Second, Pope John XXIII, Pacem in Terris (1963), an encyclical addressed to “all men of good will” denouncing the nuclear arms-race and summoning all to work for peace. Third, Pope John Paul II, Ut Unum Sint (1995), re-enforcing the Catholic Church’s commitment to the cause of Christian unity, and inviting responses from Christians about how to make the Petrine ministry a more effective service of unity for all Christians.