From www.mutopiaproject.org

"O Come, All Ye Faithful" is a Christmas carol that has been attributed to various authors, including John Francis Wade (1711–1786), John Reading (1645–1692), King John IV of Portugal (1604–1656), and anonymous Cistercian monks. The earliest printed version is in a book published by Wade, but the earliest manuscript bears the name of King John IV, and is located in the library of the Ducal Palace of Vila Viçosa. A manuscript by Wade, dating to 1751, is held by Stonyhurst College in Lancashire.
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