MLA does not have a citation style for Encyclicals or other official Church documents. This is a commonly used format:
Hard copy: Latin title (if applicable), English translation. Promulgator. [Publication city]: Publication Date.
Online copy: Latin title (if applicable), English translation. Promulgator. [Publication city]: Publication Date.Web. Retrieval Date. <web address>.
Examples
Hard copy:
Casti Connubii: Encyclical of Pope Pius XI on Christian Marriage to the Venerable Brethren, Patriarchs, Primates, Archbishops, Bishops, and other Local Ordinaries Enjoying Peace and Communion with the Apostolic See. Promulgated by Pope Pius XI. [Vatican City]: 1930.
Online copy:
NOTE: MLA no longer requires the use of URLs in MLA citations. Because Web addresses are not static (i.e., they change often) and because documents sometimes appear in multiple places on the Web (e.g., on multiple databases), MLA explains that most readers can find electronic sources via title or author searches in Internet Search Engines.
For instructors or editors that still wish to require the use of URLs, MLA suggests that the URL appear in angle brackets after the date of access.
Casti Connubii: Encyclical of Pope Pius XI on Christian Marriage to the Venerable Brethren, Patriarchs, Primates, Archbishops, Bishops, and other Local Ordinaries Enjoying Peace and Communion with the Apostolic See. Promulgated by Pope Pius XI. [Vatican City]: 1930. Web. 16 September 2012. <http://www.vatican.va/holy_father/pius_xi/encyclicals/documents/hf_p-xi_enc_31121930_casti-connubii_en.html>.
In-text citation:
It is generally agreed that well known texts can be cited simply by customary Latin title (or the first few words), followed by what paragraph is being used.
Example:
(Casti Connubii par. 4)
