Take a look at this video which answers a lot of questions but I have a couple of points to make.
a) Look for the sources. Anyone can make a claim about anything but for it to be true we have to be able to trace a source right back to the ancient world. Something which few to none of the pagan origin claims about Christmas can do. And then there also has to be an unbroken chain back. So if there was, say, (and note here that there is not) an instance where someone decorated a tree in their home in an ancient source to celebrate a god or festival you would need to be able to show that this practice was passed down through the generations till Luther used if for Christmas. If the chain is broken then the proof goes away. Luther can easily have started something new which is not based on any pagan practice. Although it seems that Luther’s Christmas tree practice was a new invention.
b) It is really hard to get any information at all about the ancient world but that doesn’t give anyone the right to make stuff up where the information is missing. I might also add that because one historian says something does not prove it, they might have been mistaken or just plain wrong.
c) Please do not believe something because it is critical of Christianity but instead look into the sources.
Anyway, here is the video