Thomas Aquinas' Summa Theologica can be cited as a footnote or endnote in a variety of similar ways. As with many ancient or medieval texts, it is prudent to give the "internal reference" and not just the page numbers of the "external reference" (which is what page numbers of translations or reprintings are). "ST" is the abbreviation followed by the appropriate Part, Part of Part, Question, and Article.
Footnote:
¹ Thomas Aquinas, ST I-II, q. 1, a. 1.
OR
² Thomas Aquinas, Summa Theologica, Ia2æ, Q.1, Art. 1.
Bibliography:
Aquinas, Thomas. Summa Theologica. Edited by Thomas Gilby, OP. 60 vols. Cambridge: Blackfriars, 1966.