I'm sorry, but I really don't get what you mean by saying you don't think that's how it goes. Everything you wrote looks like it agrees with my quote.
Just to be clear, all I was saying is atheism and theism relates to what people actually believe with regards to the question, "Do you believe there is a god?", while agnosticsm deals with the separate, albeit related, concept of how much, if at all, anyone can know about the existence of a god or gods. That's what you were saying too, right?
One thing I will add though is I don't agree that Christians necessarily need to know there is a god in order to believe there's one. As far as I'm aware, the christian belief system doesn't require certainty (although a good many fundamentalists do claim that). I'm sure a lot of christians would classify their faith as a strong belief that's doesn't require, or is held in spite of a lack of complete certainty. (e.g., accepting the bible is true on faith, even without empirical evidence to support that stance).