Latin: "Adeste fideles, laeti triumphantes, venite, venite in Bethlehem." English: "O come, all ye faithful, Joyful and triumphant, O come ye, O come ye to Bethlehem."
The first American printing of this piece (A Latin version of c. 1803) subtitles it "The favorite PORTUGUESE HYMN On the NATIVITY," but there is no particular reason to consider it Portuguese; according to Scholes in _The Oxford Companion to Music_, this title derives in fact from the Portugese Chapel in London.
The piece is believed to have been composed in the early 1740s by John Francis Wade, who also wrote the Latin words. Scholes reports an Irish manuscript of the tune dated 1746, and a variation on the theme was listed as an "Air Anglais" in the French Vaudeville "Acajou" in 1744. The rather loose English translation by Frederick Oakley appeared in 1852, based on Oakley's earlier 1841 translation.
Fuld gives details on other possible sources for both text and tune; all are possible, but not particularly likely. Substantiating details are lacking.
Recent scholarship has brought an interesting twist on this history. According to the _Penguin Book of Carols_, there are six manuscripts of this in the handwriting of John Francis Wade. The one of these thought to be oldest contains a reference to "regem nostrum Jacobum" -- "our King James," i.e. the Jacobite Old Pretender. And, of course, "regem angelorum" is quite close to "regem Angliorem," "King of England." There are also hints of Catholic practice in this manuscript. Whether all this really amounts to anything is, of course, an open question. - RBW