The chorus of the song has been around for generations, but it became a traditional Christmas song around the turn of the century when John W. Work wrote the rest of the verses.
What I didn't realize until I researched this song is that the chorus grew out of one of my favorite verses, Isaiah 40:9:
Get you up to a high mountain,
O Zion, herald of good tidings;[a]
lift up your voice with strength,
O Jerusalem, herald of good tidings,[b]
lift it up, do not fear;
say to the cities of Judah,
“Here is your God!”
So, my friends, get you up to a high mountain, lift up your voice, and let the people know that Jesus Christ is born.