MLA

Dionysius, the Areopagite, Saint, active 1st century. De caelesti hierarchia, etc. [between 1600 and 1610]. https://collections.library.yale.edu/catalog/15499931.

APA, 6th edition

Dionysius, the Areopagite, Saint, active 1st century. ([between 1600 and 1610])[De caelesti hierarchia, etc]. https://collections.library.yale.edu/catalog/15499931.