I really hate to beat a dead horse, but the United States of America is a secular nation; meaning we have no official state religion.  In case you’re confused, that’s a good thing.  It means that you can’t be harassed if your religion doesn’t jibe with the state’s.  You’re free to follow whatever beliefs you hold (or don’t hold, as the case may be), and nobody can tell you otherwise.

You absolutely can say “Merry Christmas”.  Go ahead, say it right now.  I’ll wait.

Did anybody break down your door to arrest you?  Probably not (and if they did, it certainly wasn’t because of your choice of holiday greeting).  You’re free to run down the streets shouting “Merry Christmas” as loud as you please, subject to your town’s noise ordinances.  You can say it to your postman; you can say it to a cashier; you can say it to…