Copied:I doubt you wish Muslims a "Happy Prophet's Birthday" anyway.
Christmas itself is derived from the words Christ's Mass and or worship in his name.
Wishing someone a Merry Christmas is greeting someone with hopes of happiness in Christ's name and memory of his birth.
For those that do not observe or celebrate the religious meaning of the holidays, a Merry Christmas is a traditional well wish and greeting for an enjoyable holiday and season.
In either instance, I happily return the greeting with the same and a smile.