Kenneth Sheppard has an interesting article in Patrol magazine about the history of the word “atheism.” He notes that the term used to mean something much broader, that it was used to describe anyone who was a cultural non-conformist, whether religious, political or sexual.
Between 1550 and 1750 early modern Europeans used the word atheism in a much more diffuse set of ways than we do today. When Cicero’s De natura deorum was translated in the seventeenth century, for example, there was often little difficulty in equating Diagoras’ or Theodorus’ impious unbelief as atheism – doubt about the Greek gods was often understood to be a rejection of religious belief altogether. When it first entered the vernacular languages of Europe in the early modern period, the word atheism implied not simply an intellectual denial of God’s existence, but almost any non-orthodox understanding of God and any non-orthodox practices which were taken to imply a denial of God’s existence – deviants, whether religious, social, or sexual, were conveniently portrayed as inversions of Christian belief and practice. Very often atheism was a synonym for a disturbing “other”: in early modern England accusations of atheism were closely linked to anxieties about the “foreign imports” of so-called Machiavellianism (Italy) and libertinism (France). Atheism was thus a rhetorically polemical term used in a period of intense confessional dispute.
In other words, as he defines it, atheism was simply a question of practice — a failure to act the way the dominant society told you to act made one an atheist, regardless of what one actually believed about a god or gods. The whole thing is worth reading, if you find that sort of question interesting.