The Lord to whom we cry for mercy with the Kyrie is our Savior, Jesus Christ, who was born into in the flesh. Therefore, we join the angels who sang to the shepherds at His birth and sing the Gloria in Excelsis, which is Latin for “glory in the highest.” In this hymn of praise we acclaim and extol the Son of God who humbled Himself to be our brother and now, as our Savior, reigns over us from the right hand of His Father (Luke 2:14).
The biblical reality of Christ’s incarnation, recorded in Luke’s gospel, is exactly what happens in the Divine Service—Heaven and earth are joined together in peace as the Church is gathered around the presence of Christ in His Word and Sacrament.
While it didn’t come into common use until the middle ages, the Gloria in Excelsis first appeared in our liturgy in the fourth century as part of morning and evening prayer services. However, it is said to have been used at a Christmas service in Rome even before 136 A.D.