"The word "atheism" is a direct cognate -- in fact, it is a transliteration -- of the Classical Greek word atheos (here, written with English letters). Its meaning, as demonstrated in the writings of Aeschylus and Diogenese Laertius, is best expressed as: "one who disdains or denies God or the gods and their laws." (See Bauer, Walter. Greek-English Lexicon. 2nd edition. Chicago: University of Chicago Press, 1979. p.20).
In Greek the article "a" can certainly mean "without" or "lacking" in the passive sense and when related to passive verbs, but in the case of the Greek noun atheos "a" conveys the active sense of "reversal of essence" or "opposite of condition" or "inversion of meaning." Hence, in this case, it means the bipolar opposite of its root-word theos. If "theism" is the belief that deities exist, then "atheism" is the belief that no-deities exist. Please note the place of the negation particle: when applied to nouns it should always be linked with the object, not an implied predicate; that which is believed IN is being negated, not the act of believing which is implied in the noun. In other words, and as a matter of simple Greek grammar, an atheos is one who denies the existence of a specific deity or of deities in general. Since "atheism" and "atheist" are derived from this noun, their meaning in English should follow suit. It is, hence, a misunderstanding of Greek morphology for the act of believing to be negated by the linguistic particle "a."
This analysis is supported by the word's usage in Greek literature. Essentially, it is rarely (if ever) used of a simple failure to acknowledge deities; rather, it is almost always found in the active sense of direct opposition to such beliefs, or (most often) a particular expression of such beliefs. For example, when used of Christians by Roman authorities and other Greek writers (and it was) it generally referenced their active denial of the deities of other religions ... a practice for which early Christians were labeled "atheists" by their political and religious opponents. It didn't matter that such Christians held theistic beliefs regarding their own deity, what mattered was their refusal to be ecumenical and at least passively accept the existence of other deities. Their refusal to do this -- their active denial of the existence of other gods and, particularly, their refusal to at least offer the nominal sacrifice to the deity of the Roman Emperor -- got them branded as "atheists." As a penalty for such a serious breach of cultural and political etiquette, these early Christians were sometimes severely persecuted.
While atheists will assert their identity however they wish, their analysis of the construction of the word "atheism" as being simply a passive "without god-belief" is linguistically invalid. True, the sense of passive negation is, indeed, one which the particle "a" can convey; however, that particular sense is foreign to the grammar and historical usage of the noun atheos."
http://www.errantskeptics.org/AtheismMng.htm