While I agree with Willmatter's answer, I would like to add some information to the subject.

The expression for merry Christmas in Norwegian, "god jul", has nothing to do with Christianity etymologically, it is derived from the pagan midwinter celebration of Yule: http://en.wikipedia.org/wiki/Yule

And while Norway is officially a Christian country, current surveys show that under half of all people here actually see themselves as christians, and the ones who do are usually very relaxed about it.

So you have an expression that is originally pagan, used to denote merry Christmas in a country where under half the population is actually Christian, and where politically correct versions of the expression are rarely used. Therefore I would also suggest using "god jul", unless maybe you're addressing a particular minority, in which case I'd use whatever they prefer to call it.

Merry Christmas can be seen as a christian term. But in norwegian we use God jul (good yule).
Jul (yule) is a neutral form, and refers to a winter festival. We often use the scandinavian nisse/tomte and Santa Claus as decorations and on our christmas cards.