'Church key' refers to various kinds of bottle and can openers. One prototypical variety of church key is made from a single piece of pressed metal, with a pointy end used for piercing cans and a rounded end used for openning bottles. The widget used to peel back the lid of sardine cans is also sometimes called a church key.
Some of them look a bit like keys. However, church key may just be a euphemism, since it is used for opening beer bottles and not churches; cf. the Word Doctor's discussion of the term[1].
Although the phrase seems to have come into use in the twentieth-century, some explanations give it an almost mythic significance; eg:
In Medieval Europe, Monks and Nobility were the only brewers. Lagering Cellars in the Monasteries were locked, as the Monks guarded the secrets to their craft. The monks carried keys to these lagering cellars on their cinch - or belts. It was this key from which the "Church Key" opener gets its name.
Fifty years of Polish jokes at Sons of Italy banquets, Italian jokes at the Polish-American clubs, trying the new material at the disreputable Chinese restaurant under the highway where the patrons drank scorpion bowls and blurting the word "fuck" could bring down the house.
Small audiences, the comfortable rhythm, a churchkey hanging around his neck to open the beers he'd ask the prettiest girl in the audience to fetch him from the bar.
He popped the bottle open with the churchkey around his neck with a flourish, the beer as cold as a river baptism.