The first reference I can find in the OED to "Merry Christmas" is from 1534. This date very roughly corresponds with the English Reformation and Henry VIII's breach with Rome.
From that time the idea of a "Merry Christmas" seems to take off with several entries in the 17th century. But it cannot, surely, have been protestants, let alone puritans, who promoted the idea of a 'Merry Christmas', since the word merry suggests at least mild intoxication.
Charles II (1630–1685) was known as the Merry Monarch, a reference to his lifestyle, many mistresses, and his annulment of the puritan laws instituted by Cromwell.
Until references to Christmas became politically incorrect, it was interestingly in America (the great puritan bastion), that "Merry Christmas" was more frequently used — Britons preferring the more sober sounding 'Happy Christmas'.
Does anyone know anything about the history and politics of "Merry Christmas"? Prior to let's say the 1970s, when secular symbols and "Season's Greetings", or "Yuletide greetings" start to take over, does anyone remember any strong movement against a notion of 'Merry Christmas', on the grounds that it imported a decadence into the religious festival?