I would imagine that you could be. I am the type of person you talked about. I would say that I am jewish, and I might even go to synagogue on high holidays. At the same time, I absolutely don't believe in God, or that there is a personified higher power. I am jewish because of the cultural aspects.

If you asked me, "Are you jewish", I would say yes, and if you asked me "Are you an atheist", I would say yes.

I see no reason for a christian to be any different. Maybe he wants to celebrate Christmas, or go to church and be part of a community of people similar to him. This doesn't necessarily mean that he believes in God or Jesus.