It's ok to say Merry Christmas again.
"Merry Christmas" is not a command or a statement of abstract fact. It's a tiding. It happens to be a glad tiding. There is, of course, no dearth of not-glad tidings that people bestow on others. Just as there's no prohibition on one's delivering sour tidings, there is not nor ever was in the U.S. a prohibition on offering glad tidings, most especially that of "Merry Christmas."
Folks who think there was ever a time in the U.S. when they could not say "Merry Christmas" should examine their own constitution/psyche rather than looking for external reasons why they felt that way.