In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God; all things were made through him, and without him was not anything made that was made. In him was life, and the life was the light of men. The light shines in the darkness, and the darkness has not overcome it.
And the Word became flesh and dwelt among us, full of grace and truth; we have beheld his glory, glory as of the only Son from the Father. John 1:1–5, 14 (RSV)
It is not a Christmas hymn. It resides under the 'Trust & Guidance' section in the ELW. It does sing to the wonder of God's purpose being fulfilled.
'My life flows on in endless song; above earth's lamentation,
I catch the sweet, though far off hymn that hails a new creation.
No storm can shake my inmost calm while to that Rock I'm clinging,
Since Christ is Lord of heaven and earth, how can I keep from singing?
ELW - 763
The opening of John's Gospel is a poem, a hymn to the Incarnation. When we cannot find the words adequate to express God's mercy and purpose, we write poetry, we sing. Jesus is Emmanuel, God with us, God in human flesh, true God, Son of the Father from eternity and true man, born of the Virgin Mary. Jesus is human in every way, completely and perfectly human, yet without sin. Jesus is the only human being to have lived. We are broken and distorted creatures, human flesh in need of redemption.
St. John writes, sings, of the eternal God who is the Word by which all things were made who is born of Mary, to be one of us. He is the light of the world, the maker and shaper of the cosmos, yet is of the same frail flesh as you or I.
We can imagine, we think, a God who creates then stands at a distance to see how His creation will develop. We prefer a God who keeps His distance. The Father will have none of it. The Word becomes flesh, is born, and lives among us as one of us.
St. John did not see the glory of the new-born Jesus. He did see the glory of the Risen Lord Jesus, who has spread the light of God throughout all created things. St. John sang the beauty, the glory, the joy beyond our telling of the Incarnation. God has become flesh, lived among us, died for us, and risen to redeem us all. How can I keep from singing?