Welcome mrose.
You don't have to put on an appearance for anyone, don't try to fake Christmas. We did that for our first one, the biggest deal was putting up and decoration the tree, after that we were like, uh, now what. This is our 3rd one, and we have our traditions already: decorating cookies and painting our own ornaments, you know fun stuff together.
And dear, you may feel the way you do now for several months, not just about Christmas. You're relatively newly married AND have left your childhood religion. There's confusion for everyone at different points in our lives about who we are and what we're supposed to do next with our lives. It's ok.
We're glad that you're here, and that you're at this important point in your life. Really, it's all good.