This is a seemingly-contradictory but subtle possibility based on the distinction between personal belief and objective belief. A personal belief is an assertion about how we feel about something. An objective belief is an assertion about reality. One can know something without having a desire to acknowledge it, and vice-versa.
Of course, it is much easier to disbelieve in an existant when it is perceptually remote from us, such as gods. It would be a sign of insanity to disbelieve in something immediately perceivable, such as one's toaster. But the remoteness of the concept of gods permits this distinction.
To come back to our distinction, while one may know that strong-atheism is true - i.e. that there is no god - one might still be a theist because he is comforted by this option (the question of whenever this comfort really exists is another subject, which I intend to write about also).
Likewise there is no reason why someone who believes that Christianity is true - i.e. that the Christian god exists and that Jesus is our saviour - cannot be an atheist. This is so because atheism is nothing but a lack of belief, not an assertion about reality.
As far as I know, this question has not been examined, and would deserve more thought - not for the sake of reason, but for the sake of proselyting to Christians. We tend to think of Christianity and atheism as mutually-excluding options, while logically speaking, the issue is not clear-cut.
I realize, however, that other reasons have been proposed to justify atheism in a Christian context of knowledge. One of them is the problem that, from the point of view of the Bible, the Problem of Evil and the existence of Hell, the Christian God seems radically evil.
Given that fact, it can be debated whenever we would be morally justified in obeying such an evil god. For example, we might expect that an evil god would not hold his promises, and that therefore obedience is not desirable. Indeed, the Christian God has a bad track record in fulfilling his promises.
Since I have admitted that this concept would exist mainly for the purposes of apologetics, I am forced to recognize that this position of "Christian atheism" may be seen by Christians as a rebellion against God. I am not sure what would be its ultimate value. But it seems to me that its logical existence is beyond doubt.
