One of the wonderful benefits of knowing music theory well (including chords) is the choice you have in key selection. You’re not limited to playing a song in just one key — you can use several if you wish. Take a look at the video below on Adeste Fidelis:

The text to the Carol O Come All Ye Faithful was originally written in Latin (Adeste Fideles) and was intended to be a hymn, which it is, but has also come to be one of our beloved Christmas Carols about our Lord.

O Come All Ye Faithful
Joyful and triumphant,
O come ye, O come ye to Bethlehem.
Come and behold Him,
Born the King of Angels;
O come, let us adore Him,
O come, let us adore Him,
O come, let us adore Him,
Christ the Lord.

O Sing, choirs of angels,
Sing in exultation,
Sing all that hear in heaven God’s holy word.
Give to our Father glory in the Highest;
O come, let us adore Him,
O come, let us adore Him,
O come, let us adore Him,
Christ the Lord.

All Hail! Lord, we greet Thee,
Born this happy morning,
O Jesus! for evermore be Thy name adored.
Word of the Father, now in flesh appearing;
O come, let us adore Him,
O come, let us adore Him,
O come, let us adore Him,
Christ the Lord.