This is a fifteenth-century Latin carol, with a tune of the same date. They wrote much sprightlier melodies back then than they do now. 

He whom joyous shepherds praised,
When the angel’s song was raised
Bidding them be not amaz-ed,
Heav’n’s all-glorious King is born.

He whom sages, westward faring,
Myrrh and gold and incense bearing,
Worshipped, bowing low before him,
Reigns as King this happy morning.

Now rejoice with Jesus’ mother;
Praise her newborn son, our brother;
Angels vie with one another,
Praising him beyond the sky.

Sing to Christ, the King who reigneth,
Yet of Mary manhood gaineth,
Born our God; let us adore him:
Glory be to God on high.