Many of those people would indeed not describe themselves as Christian anymore, or they would use terms like non-practicing Christian, cultural Christian, and the like. They'd probably still have Catholic weddings and funerals, perhaps still baptize their children. There would certainly be evidence of a Christianity-inspired morality and worldview in their lives - just not prayer or any actual belief in God. I and many others of my generation were baptized and confirmed (those rates are dramatically lower among today's kids, though), I even served as an altar boy, but that doesn't mean we ever really believed in God or prayed outside of church.

I normally would describe myself more as agnost than as Christian, but in certain contexts among people who aren't so familiar with concepts like agnosticism, humanism and the like, I'd probably be happy to simplify things by calling myself a non-practicing Christian.