To find the answer we must examine the Latin text in its original order.
Originally a Latin hymn, “O Come, O Come, Emmanuel” was written between the 8th and 9th centuries. Although it was lost for many years, it was rediscovered by John Mason Neale in the appendix of an early 18th-century manuscript. Neale, who was a hymn writer and translator of early Greek and Latin, translated the words to English and published it in his influential collection Mediaeval Hymns and Sequences (1851).
Monasteries traditionally sang the “O antiphons” on the seven days prior to Christmas Eve, when the 8th antiphon, “O Virgo virginum” (“O Virgin of virgins”) would be performed before and after the Canticle of Mary.
There have been many translations of the text, which sometimes mix up the order of the antiphons. It is in their original order, however, that we find a curious riddle that reveals its true meaning when sung on Christmas Eve.
The antiphons, in their original order, begin as such:
O Sapientia (Wisdom) O Adonai (Lord) O Radix Jesse (stem or root of Jesse) O Clavis David (key of David) O Oriens (Dayspring) O Rex gentium (King of the Gentiles, or nations) O Emmanuel (God-with-us)
Let’s examine the second word in each antiphon, the word after the O. If the first letter of each word is taken and put together it creates S-A-R-C-O-R-E. On its own this is nonsense, but if read backwards the letters form a two-word acrostic “Ero Cras,” which means “I will be here tomorrow.”
All of the Latin/Hebrew words describing the messiah come from the Old Testament, aside from Emmanuel, which is found in Isaiah and Matthew, who practically quotes Isaiah verbatim. The “O Emmanuel” antiphon, which represents the fulfillment of the Old Testament prophecies, was traditionally saved for Christmas Eve, when those clever enough to discern the riddle would see it revealed.
UMCDiscipleship.org points out that J.R. Watson, a British hymnologist, provides a context for the antiphons:
“The antiphons, sometimes called the ‘O antiphons’ or ‘The Great O’s,’ were designated to concentrate the mind on the coming Christmas, enriching the meaning of the Incarnation with a complex series of references from the Old and New Testaments.”
So it appears that “O Come, O Come, Emmanuel” was more like a meditation than a hymn, one that reflects the mysteries of faith with a riddle, which develops further as Christmas draws near. Today, when singing the antiphons in their original order, Christians get to take part in a sacred Advent ritual at least 11 centuries old.