There is no Christian culture.
That's not true at all. There's Christian culture just as there's Jewish culture, Hindu culture, Buddhist culture, etc. We have our in-group traditions and practices that provide a strong sense of identity, just as you Jews do. These aspects are readily apparent especially in Catholicism and Orthodoxy. Just like the Jews, Catholic and Orthodox families pass down their religious traditions. Over time, it becomes inseparable from the wider culture. Try to separate Catholicism from Italy or Russian Orthodoxy from Russia. It really can't be done. In Catholicism, we also have the concept of a lapsed Catholic or nominal Catholic, which means that you identity with Catholicism as a culture and tradition but don't believe in it. That's much the same as a secular Jew.