How do I reference an encyclical letter (Papal document) in Chicago style?
The following example is taken from the University of Notre Dame Australia's Referencing Guide for Chicago Footnotes:
First footnote citation
5. John Paul II, Evangelium vitae, Encyclical Letter, Vatican Website, March 25, 1995, http://www.vatican.va/edocs/ENG0141/_INDEX.HTM, sec. 97.
Second and subsequent footnote citations
9. John Paul II, Evangelium vitae, 34.
John Paul II. Evangelium vitae. Encyclical Letter. Vatican Website. March 25, 1995. http://www.vatican.va/edocs/ENG0141/_INDEX.HTM.
Please refer to the UNDA Referencing Guide for Chicago Footnotes (which is the style used by the School of Philosophy & Theology), under Religious & Classical Works > Footnote & Bibliography examples > Papal Documents.