Amazing how this topic seems to turn into a bitter, nasty argument every year. And of course, nothing about it ever changes. People will say whatever they're comfortable with.
As to the question of what is wrong with being a true believer, here's what's wrong. A "true believer" is defined by Eric Hoffer as a dyed-in-the-wool fanatic, someone who staunchly believes that
his/her beliefs, and only those beliefs, are valid, correct, and worthy of respect from other people. This can pertain to religion, political beliefs, race, gender, maybe even sports teams!
And those beliefs, held by people wearing blinders, have down through the centuries led to wars, torture, physical and mental cruelty, prejudice, refusal of civil rights, imprisonment, censorship,
insistence on allegiance to rules and principles which hold no truth for a lot of people, but they have to pretend to go along anyway, out of fear for their lives.
I think there's a whole lot wrong with that. The swaggering contempt for other people and their beliefs is one of the worst traits of human nature.
So, saying "Merry Christmas" out of a joyous desire to share one's own Christmas cheer is a nice thing to do; certainly there's nothing wrong with that. Saying it arrogantly, along with a threat that
you'd better like that greeting, or else you "won't like my answer" is definitely not all right; in fact, it demonstrates the worst of human nature.