since Xmas comes from the birth of Appolo. I found a explanation by a theology scholar -- since nowadays the Roman polytheism distincted, nobody regards celebration of Xmas as paganism; therefore it's acceptable for christians.

If we follow this logic, since most people just think xmas as a common holiday (geez, what's the meaning of holi), it's not so misleading if you say merry xmas. Unless you are very PC.