Gosh, I can hardly keep up (and apparently the LEO server is having trouble as well). So this is going to be long, but I hope you can bear with me.
First, a couple of quick clarifications.
>>Wir wissen nicht ob es Jesus gab
Actually, that's not true. We know that he was a real, historical person with as much certainty as we know it about anyone who lived millennia ago. We also can't prove that, say, Socrates existed, but all the evidence certainly points to it.
>>niemand weiß wann er wenn überhaupt geboren wurde
That's also a bit inaccurate. Based on written sources, historians can pin it down fairly closely for something so long ago. We do know that it probably wasn't in Bethlehem, but we also know that it was probably around 4 BCE, plus or minus a few years -- that is, the year 0 is probably not wrong by a very large factor.
However, I totally agree that no one can say accurately that Christmas is a purely religious holiday. It's been a mixture of religious and secular influences from the very beginning, and it still is. It's a human tradition, and humans are complicated creatures, full of contradictory motives. Purity is unrealistic, paradox is rich and interesting. What's so bad about that?
But I also don't think it should be just about family and friends. If you don't see it as sacred, fine, but if you celebrate it, there should be something about it that makes you look beyond yourself, think about the rest of the world, not just about food and presents. Something, shall we say, altruistic.
And exactly that aspect is what seems to me to be missing in the practice of sending business Christmas card. Yes, Christmas in some contexts is a religious holiday, but to me, commercial Christmas cards are just not one of those contexts.
Think about it. Do most modern companies observe religious standards the rest of the year in their financial dealings? Do they give 10% of tneir profits to charity? Do they refuse to charge fees for credit or late payments, and periodically forgive all debts? If someone takes them to court and sues, do they meekly offer double the amount demanded? Do they provide generously for the health and welfare of their employees, ex-employees, and employees' dependents? Do they bend over backwards to ensure safe, comfortable working conditions, and trade only with other companies with the same standards, even in developing countries? Do they distribute profits equally among all workers, from the CEO to the janitor? Forgive me if I doubt it.
In the modern world, business is business and religion is religion. Most companies send holiday cards for one reason only: advertising. They want to make their customers and trading partners feel good about them and continue to do business with them. There's nothing terribly wrong with that, but borrowing religious words and symbols for that purpose is really cheating, preying on people's unconscious associations.
And the more overt it gets, the more it starts to look political as well. Not unlike sending out patriotic symbols on a national holiday (which I too had never heard of) -- now that too looks like a really shameless attempt to promote the beliefs of one party, namely, the party that (IMO falsely) claims to represent God and country. It's preaching to a circle of insiders, celebrating 'us' against 'them' -- actually the total opposite of what religion is supposed to be about.
For me, the words Merry Christmas themselves aren't that big a problem, because in most Western countries, Christmas is now also a secular winter holiday celebrated in some form or other by many people who are not Christian or not even religious. Wreaths, trees, winter scenes, all those are acceptable neutral images. And for a message, I quite like nja's suggestion (#60) as a way of distinguishing between the greeters and the recipients: we're celebrating A, and we would like to wish you B. But the farther a business card goes in the direction of mangers, shepherds, angels, Bible verses and so on, the more often and prominently it uses the word Christmas, and the more obviously it avoids more inclusive greetings -- the more hypocritical it all starts to look.
In the end, there's still no escaping that Christmas cards from businesses are basically impersonal, materialistic, wasteful. As Sahe said in #12, why spend so much money, paper, and labor on something that most people will just toss in the trash? Why kill the trees, why stuff the landfills? If companies really wanted to be religious, they could skip buying cards altogether and give the money to a homeless shelter. Or they could at least suggest that employees send individual, handwritten notes to the clients they work with, not just have a pre-printed mass mailing.