Okay, let's take this logically.
1. There are people who exist who call themselves Christian who do not believe in God. Nobody is denying this, right?
2. This being the case, there are two basic responses:
- dismissal of the claim - i.e. they can't be Christian because no true Christian would say such a thing (the logically fallacious 'no true Scotsman' argument), or
- taking the claim seriously at some level.
3. If you assert that they are a priori not Christian whatever they claim to be, you are dismissing a claim of belief and identity, whilst asserting your own right to make such a claim and be taken seriously. This is hypocrisy.
4. If you dismiss their claim by recourse to scripture, some institutionally agreed script, speech-act, etc. you are relying on an already mediated understanding of 'Christianity', which requires that we accept that Christianity must be defined in a certain way - exactly the way in which the person making the claim to be Christian is already rejecting. This might satisfy you if you are fundamentalist or an adherent of one or other organised Christian sect etc. but it does not satisfy logic.
If you think that there can't really be religious people who think this, then I suggest you look up theological noncognitivism. I would also suggest that there are multiple accounts of Jesus Christ that do not see this particular figure in the same way (the story of Jesus in Islam is just one example), and it is possible, if unusual, to separate the question of Jesus's divinity from other religious implications and from his social and political teachings (or what is attributed to him, since the words of Jesus are nowhere recorded directly).
I should point out that I am not saying that this is desirable, good, better than other kinds of Christianity, just that it is possible and people who believe these things exist and call themselves Christian, and thus being a Christian who does not believe in God is not something you can deny except by recourse to the doctrine of a particular institution that would claim in some way to represent or define the idea of 'Christian'.