When somebody sends you a text message wishing you a Merry Christmas, replying "Who is this?" is simply NOT an option, okay?
Obviously that person thought enough of you to save your name/number and include you in on a depersonalized bulk text message. Just text back "thanks" or "you too!". no need to make the texter feel like a dejected loser and quietly cry himself to sleep.
Not that that ever happened to me five times or anything. Nah, nope, not me.