Neither do I, but some people do. And they use the bible as their material and belief, for making their points regarding what God/Jesus said and didn't say, how people ought to believe, behave and so on.
Arguing against that, by simply saying that you don't believe in God, makes little sense to them (and me
) Because a lot of times I disagree with the conclusions that they get to when they quote the bible, so to me, they use the material wrong and that is what im arguing against.
Not really a lot different than if you quote Lord of the Rings and claims that Gandalf weren't actually a wizard or something, based on this and this, then I would say the same, that you were using the material wrong. Even though non of us actually believe that Gandalf exists.