The distinction between a believing Christian and a cultural Christian is dubious, because religion is culture. Belief is not, or not just, an invisible thing in one’s head — it takes the form of culture. A lot of people are not sure if they believe, or are not sure how to articulate their half-belief, but sometimes take part in Christian worship — even if it’s just singing the occasional carol, as Dawkins himself enjoys doing. Dawkins wants to categorize such people as merely cultural Christians, like him, not believing ones. But there is no clear distinction.  

In Dawkins’ view, it is harmless and even healthy to participate in Christian culture a little bit, for there is a “beauty” and “decency” to it, but it is deeply mistaken to cross the line into “belief.” But there wouldn’t be any Christian culture if there weren’t plenty of committed believers, and there is no clear line between mild participation and tentative belief. In practice, someone who values Christian culture, and sometimes dips a toe in, and is more nuanced and honest than Dawkins, often admits to believing a bit too. I’m sorry if it confuses the sciencey mind, but religious belief just isn’t black and white.