Ask people of a non-Christian faith if they agree. All the replies here saying Christmas isn't Christian seem to be from people with a Christian upbringing, even if they are atheist or "secular".
Even if no one of a non-Christian background celebrated secular Christmas, “celebrating secular Christmas” still wouldn’t suffice to identify someone as Christian, but rather as either Christian or “from a Christian background”.
And of course lots of people from Jewish, Hindu, secular etc backgrounds also celebrate secular Christmas, as many have attested in this thread.
Maybe that is what you're debating. It was only one example that I gave in my original post.
It seems to me that a lot of people who grew up in the dominant culture of the US are jumping to defend Christmas as a wholly secular thing. It would be interesting to watch if it weren't so typical.
You know, because the context of the thread is characters’ religious identities.
Maybe in US it’s more strongly christian. But not necessarily everywhere.