Bottle openers are referred to as "churchkeys" because the original design resembled an old fashioned church key used by monks to open the doors of the church. The first churchkeys were invented to open "crown cork" glass bottles, although the term churchkey was not coined until later.
Although the designs have varied over the years, the original design was visually similar to that of a simple key. When the first flat-top beer cans began production in 1935, an opener that would puncture the top of the can was needed.
Made all of one piece, these new metal openers had a large loop on one end for opening bottles and were pointed on the opposite end for piercing the tops of beer cans. The term "churchkey" was adopted for these openers, as their design was similar to the original item that used the name. Companies, such as the American Can Company, gave these openers away for free with their beer cans. Flattop cans went out of production by the middle of the 1970s but the openers still found use in opening cans, and the name stuck.
As of 2012, a company called Churchkey Can Company began marketing the flattop can once more, bringing the churchkey bottle opener back.