So - not race nor religion, then.Cultural.
I intentionally did not say Christian as one could follow the teachings of Christ simply because they are a good idea so would that just be cultural as well?Why not? You could argue that people who celebrate Christmas, enjoy watching their kids do the nativity play and happily sing Christmas carols, but do not believe in God, are Atheist Christians.
Anyway, not believing in god would seem to preclude one from being a member or follower of that religion.