"The pagan festival
with its riot and merrymaking was so popular that Christians
were glad of an excuse to continue its celebration with
little change in spirit and in manner. Christian preachers of
the West and the Near East protested against the
unseemly frivolity with which Christ's birthday was
celebrated, while Christians of Mesopotamia accused
their Western brethren of idolatry and sun worship for
adopting as Christian this pagan festival."
-
Christmas, New Schaff-Herzog Encyclopedia of Religious Knowledge, p. 48.
"The Mithraic
Christians actually continued to celebrate Christmas Day
as the birthday of the sun, despite the censures of the Pope;
and their Sunday had been adopted by the supplanting
faith."
- Pagan Christs, by John M. Robertson p.332