If you are using the Chicago 'notes and bibliography' style to reference St. Thomas Aquinas' Summa Theologica :
The following example about the Summa comes from the 'question and answer' section in the Chicago Manual of Style (CMOS) website, i.e. :
Q. How would you cite Aquinas’s Summa Theologica, Question 65, Article 4?
A. If you are using Chicago style, follow the form at CMOS 14.257 (14.257 Identifying numbers in classical references):
Aquinas, Summa Theologica 1.65.4.
Note that you need a part number as well as a question and article number.
Our UNDA Library page links staff and students with the complete Chicago Manual of Style (under eResources). There is also an introductory Referencing Guide with Chicago-style examples involving sacred texts and religious works: http://library.nd.edu.au/referencing/chicago/religious
Aquinas' Summa is among these :
First footnote citation
2. Thomas Aquinas, Summa theologiae, (New York: Benziger Brothers, 1911-1925), IIa-IIae, Q.17, art.6-8.
Second and subsequent footnote citations
5. Aquinas, Summa theologiae, Ia, Q. 28, art. 2, ad. 2.
Aquinas, Thomas. Summa theologiae. New York, NY: Benziger Brothers, 1911-1925. IIa-IIae, Q.17, art.6-8.