Here's an answer I wrote earlier this month which may help fill some blanks. There are a few different forms of the 'Christmas is pagan' meme, and they have distinct origins.

The most lasting form is one that was fabricated by Puritans in 17th century England, as part of a campaign to outlaw Christmas, ostensibly on the grounds that it's (supposedly) Catholic, and therefore no better than heathen. Another form of the meme developed in 19th century continental scholarship (and with a broader range of religious support, from both Protestant and Catholic scholars). Other versions of the meme can usually be regarded as tagging along for the ride.