The Latin "Adeste Fidelis" was written by John Francis Wade, an English Catholic who fled the country after the failed 1745 rebellion, and it was not translated into English until 1841.
The Daily Telegraph quoted Professor Bennett Zon from Durham as saying:
I mentioned this story to my mother the other day. She said that her own mother, an East Anglian from good Dissenting stock, had always regarded "O Come All Ye Faithful" as a Catholic hymn and disliked it as a result.
"There is far more to this beloved song than meets the eye.
"Fideles is Faithful Catholic Jacobites. Bethlehem is a common Jacobite cipher for England, and Regem Angelorum is a well-known pun on Angelorum, angels, and Anglorum, English.
"The meaning of the Christmas carol is clear: 'Come and Behold Him, Born the King of Angels' really means, 'Come and Behold Him, Born the King of the English' - Bonnie Prince Charlie!" ...
The Jacobite meaning of the carol gradually faded with the cause.
"Adeste Fideles seems to have lost its Jacobite meanings not long after Wade's last published book in 1773," said Prof Zon
So maybe its origins were forgotten more recently than Professor Zon believes.