I read it as a flowery, archaic way of saying something along the lines of "in the name of God", without needing to map it away from a modern meaning, so that's one data point for you. I don't recall hearing the phrase elsewhere, but there are lots of religious invocations along similar lines from various eras, and I may unconsciously be drawing an inference between them.

(My favorite might be "God's teeth!", although that conveys shock rather than supplication.)