I just can't wrap my head around this. I was raised Christian, but haven't been one for decades. I still kept going to church though long after my conversion just to remain undercover. I remember an evangelical preacher, who let's say wasn't exactly the most tolerant or moral person (this man was an open wife beater, and his entire congregation was 100% fine with this to give you a idea). He thought the idea of a "cultural Christian" was absurd. Loo-loo as he was, I see no reason to disagree with him.
Christianity is centered around belief. Christians define themselves as people who "believe in Jesus", believe in God, the Exodus, etc. I even had one person one Reddit once asked me if I kept to the tenets of Christianity, and then proceeded to list nothing but beliefs on history and how the universe works. Not one thing about morality or behavior.
Yes, Christians do have behaviors and practices, but they all come from how they believe the universe works. They evangelize because they believe non-Christians all go to hell. They campaign against the LGBT movement because they believe the Bible says it's wrong. They oppose the teaching of evolution because it contradicts the narrative given in Genesis (though the 'old earth' theory itself also poses some logic problems).
If you don't believe any of this stuff, I don't see how you can be a "Christian". What, are you going to be a homophobe and go around trying to convert people to Christianity when you're not a Christian yourself? Are you going to deny evolution when you have no practical reason to do so? Are you going to continue to believe in the historicity of the Exodus despite evidence to the contrary (granted, I myself kept believing that the Exodus was a historical event until rather recently, minus the supernatural bits of course).
Based on my own upbringing, I don't see how you can be a "cultural Christian". Christianity hinges too much on how you believe the universe works. If you ditch all that, how can you be a Christian? That makes no sense. What else is there to Christianity outside of its beliefs? What, are you going to be unlike other atheists and appreciate Christian art rather than blasting it simply because it was made by Christians? That's the only thing I can imagine: a "Christian" taste in aesthetics. Although I don't see why a person like that would still call themselves a "Christian" in any other capacity.