Well it's an interesting discussion. :)
The sentence I referred to is (deliberately, I think) ambiguous. It can be understood in many ways -- including the one(s) intended by the author hopefully; I'll come to that later, although quietlyleaving and slimboyfat's links already give some clues about the general framework of Lacan's problematic, which is definitely not about religious belief in the ordinary sense.
But I'll stick for the moment to the (non-Lacanian) way it has been mostly understood so far.
Atheism is conceptually tributary to theology. It receives and accepts from theology the notion of "God" it denies. And it must consider this notion fixed once and for all to have something (solid) to deny. Which means that it naturally chooses as interlocutors apologists of a traditional, orthodox or fundamentalist exoterical theology; objective agreement with them on a positive definition of "God" (the "what") is a prerequisite for the ("whether," "yes" or "no") debate. At the same time, it must ignore the ongoing work of fundamental theology to which "God" is essentially a question, an unknown "x" -- or the unknowable itself as in the old tradition of apophatic theology. How would an atheist respond, for instance, to Tillich's definition of "God" as the ground and abyss (Grund / Abgrund) of being, the "ultimate concern" of man, which doesn't exist (ek-sist), doesn't "add" to the world as a separate and additional "being"? Or to the God of Process theology (Whitehead, Cobb) which is an integral part of evolutionary and historical becoming? I don't mean there can't be an atheistic answer to such definitions of "God" (Spook, in another thread, made an excellent point about the fact that such re-definitions are not those which have been held by the vast majority of believers in history; that would be worth discussing). But still each theology would imply a different kind of "atheism".
On the other hand, the history of theology contains much atheism right from the beginning. Monotheism emerges (with Deutero-Isaiah) as the denial of gods -- not only "all but one" as is often held. As the English capitalisation implies, "God" is not "a god" at all, even if he historically assumes (and modifies) the pre-monotheistic tradition of one god (Yhwh). In a sense, atheism pursues the monotheistic agenda of anti-idolatry which is coextensive to (mono-)theology.
From this perspective, I think there is much wisdom in ATJ's suggestion of "trying to define your own stance". A lot of theologians do not believe in the "God" which atheists deny. And when atheists stop fighting against classical theism and begin expressing what they do believe they often come surprisingly close to theological concepts old and new.
Incidentally, that's precisely what happens with Lacan among many others: an atheistic thinking (by usual standards) with a deep and conscious theological structure. More about that later.