Apparently, this song was originally a Catholic hymn written in Latin (Veni, veni, Emmanuel). The modern version we are used to was written in the 1800s, but the original could be as old as the 8th century. It also might have been used as Gregorian chants or a processional for Franciscan nuns. Sweet!
Although we usually think of it as a Christmas carol, it's actually an Advent song. It's taken from Isaiah 7:14 which predicts the Messiah coming to us. Obviously Jesus has already come, but now we use Advent as a time to remember his coming as well as to look forward to his next coming.
The Latin (traditional) version.
The Sufjan Stevens (indie) version.
The August Burns Red (hard) version.