The original “Hark! The Herald Angels Sing,” written by English Methodist Charles Wesley in 1739, started as a poem inspired by the sounds of the London Church bells and was to be read on Christmas Day. Later becoming a somber tune, fitting of a hymn, it stayed that way for over a century. In 1855, William Cummings decided to set the hymn to an uplifting melody, with a slight change in the wording. He borrowed the tune of Felix Mendelssohn's cantata, Festgesang, which was often referred to as the ‘Gutenberg Cantata.’ The Gutenberg Cantata was originally written by Mendelssohn for a celebration honoring the 400th anniversary of the invention of (moveable type) Gutenberg's printing press.
It's ironic that a Christmas hymn would be made to music which was actually written to honor the machine that printed the Bible, making the printing process popular in the first place! While you're caroling this season, remember there's often a little history hidden behind the lyrics, and many did not even start as Christmas songs.