The melody to this carol was written by Felix Mendelssohn in a piece to commemorate the 400th anniversary of Gutenberg's invention of the printing press; the words by Charles Wesley, one of the
founders of Methodism.
Actually, the original words wereHark how all the welkin rings/Glory to the King of Kings. ("Welkin" means heaven.) A colleague substituted the now familiar opening lines against the protests of the author. Wesley also requested that the music put to the words should be slow and solumn. Unfortuneately (or fortuneately for us) William Cummings did not agree amd in 1855, after both Mendelssohn and Wesley were dead, he matched the verses to the current melody.