The letter 'a' as prefix usually indicates a position against the idea of the rest of the word. As a result you you would see 'a' indicating opposition to 'theism' : 'the view that all limited or finite things are dependent in some way on one supreme or ultimate reality of which one may also speak in personal terms'. (Enc Brit.) Similarly, 'a' would indicate an opposition to gnosticism the Gnostics' distinction between the unknown supreme God and the Demiurge (identified with the God of the Old Testament) who created this world (Enc Brit). Really the distinction is 'I have no belief in the existence of god' and 'I dunno, could be but maybe not'