Wesley had requested and received slow and solemn music for his lyrics, not the joyful tune expected today. Moreover, Wesley's original opening couplet is "Hark! how all the welkin rings / Glory to the King of Kings".
The popular version is the result of alterations by various hands, notably by Wesley's co-worker George Whitefield who changed the opening couplet to the familiar one, and by Felix Mendelssohn. A hundred years after the publication of Hymns and Sacred Poems, in 1840, Mendelssohn composed a cantata to commemorate Johann Gutenberg's invention of the printing press, and it is music from this cantata, adapted by the English musician William H. Cummings to fit the lyrics of “Hark! The Herald Angels Sing”, that propels the carol known today.
Hark! the herald angels sing, "Glory to the newborn King!" Peace on earth, and mercy mild, God and sinners reconciled Joyful, all ye nations, rise, Join the triumph of the skies; With th' angelic host proclaim, "Christ is born in Bethlehem." Hark! the herald angels sing, "Glory to the newborn King!"
Christ, by highest heav'n adored: Christ, the everlasting Lord; Late in time behold him come, Offspring of the favored one. Veil'd in flesh, the Godhead see; Hail, th'incarnate Deity: Pleased, as man, with men to dwell, Jesus, our Emmanuel! Hark! the herald angels sing, "Glory to the newborn King!"
Hail! the heav'n born Prince of peace! Hail! the Son of Righteousness! Light and life to all he brings, Risen with healing in his wings Mild he lays his glory by, Born that man no more may die: Born to raise the sons of earth, Born to give them second birth. Hark! the herald angels sing, "Glory to the newborn King!"