Every year I hear the same thing: Christmas is a pagan holiday, the Christians stole it from us.
That is simply not true. Many of the traditions and ways of celebrating it are pagan in origin, but that does not mean the holiday itself is. As various cultures converted from paganism to Christianity, they brought their seasonal traditions with them. It wasn't always the case that "the evil priests tricked the pagans into making their festivities into Christian ones".
As for the conversions themselves, sometimes it was at knife-point, but more often it was because it was financially or politically expedient. A king, emperor, or chieftain would convert, and then the juicy "government contracts" went to fellow Christians. And sometimes, sometimes, people converted because the faith felt right; it felt like coming home. I hear that all the time from pagans that they felt this way when found their new faith. Why can't it work in the opposite direction?
Christmas is a Christian holiday. It is a celebration of Jesus' birthday. Very likely He was not born in December, and many suggest is was moved to December 25 to coincide with an older, pagan holiday. However, from what I've seen, no one can agree on which pagan holiday. Myself, I believe that part of the reason for the move is that you just need an excuse for a bit of warmth and a bit of a party that time of year.
Christmas is also a secular holiday. It outgrew the chapel and spilled into the Victorian parlor, the city streets, and then the shopping malls. Again, I think it is partly because you need the warmth and a party in the winter.
You might celebrate Yule, Saturnalia, Alban Arthuan, or something else entirely different. You may celebrate the secular Christmas or you may dread it. But even if you don't keep Christmas yourself, your co-worker with the crucifix likely does. If you wish them a merry Christmas, it isn't because it is important to you, but that it is important to them; the wish isn't for you, it is for them. My non-American friends will wish me a happy fourth of July. It isn't their holiday, but they know it means something to me.
There are many Christians that anger me this time of year (but there are also many pagans that embarrass me.) It is easy to find enemies; if you look you will see them even where they aren't. Instead, look for friends and allies.
Tolerance doesn't begin with someone else; it begins with you. You can't demand that someone else change their attitude if you are unwilling to change yours first.