Leave your thoughts on Institutes of the Christian Religion here
in Book 1: Chapter 1: section 3: a footnote  with a series of cross references
comparing with the page image of http://www.ccel.org/ccel/calvin/institutes/Page_39.html
the PDF and text version and perhaps others have the the cross reference of Gen 17:27
while the page image ( the url above ) shows Gen 18:27 (Gen xviii, 27) the Genesis
18 passages "Who am i but dust an ashes" better follows from Calvin's text in section
3 of chapter 1 ( book one) than the Genesis 17 passage concerning circumcision.
This is likely just a typo in the footnoted cross reference list at some point in the process.