Wordorigins.org

Church key, an American slang term for a bottle opener, is first attested to in 1951, although it is undoubtedly older.

It’s so called because the bottle openers resemble the heavy, ornate keys that unlock big, old doors like those found in churches. The origin may also be related to the irony of associating churches with drinking.¹

¹Historical Dictionary of American Slang, v. 1, A-G, edited by Jonathan Lighter (New York: Random House, 1994), 422.