They want to make people feel that, if they do anything that regular people do, they are blowing their one and only chance to bring those people into the cancer as a guilt trip. You celebrate Christmas, among its being so bad in its own right, you just blew a chance to bring in others by not celebrating. They could have seen you not celebrating, wondered what was so wonderful in your religion, and joined that way--but no, you had to go and put up that Christmas tree. All it does is puts up a guilt trip.
In reality, most people are turned off by such rubbish. Putting up a Christmas tree is perfectly normal--whether or not Jesus was born on December 25, whether or not you are indeed worshiping the sun and the Devil, that is how most people think of Jesus. Now, if someone not only doesn't do it but objects to others doing it, they are not going to want to join that religion. Whether it be by speaking against Christmas music, bashing the lights that decorate the yards, or doing things to sabotage things (like disabling or, worse, ruining lights, turning off their Christmas music, etc.), you are going to get people to not want your religion. They see that, if they join, they are not going to just be able to mind their own business but they are going to have to worry themselves sick about things.
This also goes for everything else. Not joining field trips in school, not saluting the flag because of religion (as opposed to because they don't believe in their country any more), wasting all their time going door to door, dressing in dorky dresses and suits, and giving up good jobs to pious-sneer are just as bad. Usually, the witlesses will miss out on things in school--which will not draw others to want to miss out. I don't think I would have wanted to miss out on the fun things for a damn religion when I was in school, and I don't think many others want to either.