atheist - "Happy holidays" or "Merry christmas"

Yestarday me, Jack and David had this discussion during our lunch.

Topic: How should an atheist greet people for Dec 25?
-Happy holidays
-Merry christmas
-Season's greeting

Dictionary point of view

Happy holidays: In word 'holidays', 'holi' stands for 'holy'. So an atheist may have subtle difficulty when using this. Some christian believers may get offended when they hear Happy holidays instead of Merry christmas

Merry christmas: An atheist may have real difficulty when using this.

Season's greeting: Sounds perfectly fine...

Humane point of view
Greetings does not directly make things happen, but the greeting-reciepient's mind can certainly affect events.
So, while greeting, its good to use what the reciepient would like to hear.
So, I think, an atheist should use Merry christmas if it makes the recipient happy...