How did Merry Christmas get to be a Christian greeting?  'Christmas' takes at the Winter Solstice.  When I was growing up, I was told that Christmas time is a special period for people to express love and caring for one another, and there is no conflict between Santa and 'Jesus in the Manger' because they are part of the same thing, the spirit that can be called 'Santa Claus' or 'the Christ Child', or the Great Flying Reindeer'.  It's the 'Christmas spirit', whatever we call it, that has us speaking to strangers and wishing them well.  Some of us practice that all year long, some need special permission.