I just spent far too much time arguing that there is not a war on Christmas. Brought in facts like Jesus was most likely born in the summer (someone else pointed out that he may never have existed at all) and Christmas being a co-opted pagan holiday. I’m new at this atheist/agnostic thing and it’s exhausting af! They are all emotion and no reason. I can’t believe I used to be like that.
The amazing thing is that hundreds of millions of people think like that. Literally hundreds of millions of humans can't get their minds past the imaginary.
You don't owe them an explanation, and you don't owe them enlightenment. In fact, it's hard to say they shouldn't proselytize you when you are, effectively, proselytizing them. Let them be as they are, so long as they leave you alone. If the people you are engaging with are trying to correct or convince you that you're in error, put up healthy interpersonal boundaries and enforce them, and then find people who don't do that to you to spend more time with.
In my opinion, they feel that there is a war on Christmas because the facts are overwhelmingly mounting against them and, deep down, they know your right. They can sense their crutch being taken away. They realize that it is only them and no God, or son of said God, there beside them and they cannot function without that thought. It's too engrained into their life to live without and that terrifies them.
Sweet spirit, save your strength and sanity: those who would respond in a constructive fashion to your challenging facts (true as they may be) are few and far between.