Even though we disregard philosophical atheism, the definition is somewhat narrow; for in antiquity mere denial of the existence of the gods of popular belief was the only attitude which was designated as atheism. But it has the advantage of starting from the conception of the ancient gods that may be said to have finally prevailed. In the sense in which the word is used here we are nowadays all of us atheists. We do not believe that the gods whom the Greeks and the Romans worshipped and believed in exist or have ever existed; we hold them to be productions of the human imagination to which nothing real corresponds.
1-2

Atheism and atheist are words formed from Greek roots and with Greek derivative endings. Nevertheless they are not Greek; their formation is not consonant with Greek usage. In Greek they said atheos and atheotes; to these the English words ungodly and ungodliness correspond rather closely. In exactly the same way way as ungodly, atheos was used as an expression of severe censure and moral condemnation; this use is an old one, and the oldest that can be traced. Not till later do we find it employed to denote a certain philosophical creed; we even meet with philosophers bearing atheos as a regular surname. We know very little of the men in question; but it can hardly be doubted that atheos, as applied to them, implied not only a denial of the gods of popular belief, but a denial of gods in the widest sense of the word, or Atheism as it is nowadays understood.
5

Atheism, in the theoretical as well as the practical sense of the word was, according to the ancient conception of law, always a crime; but in practice it was treated in different ways, which varied according to the period in question and according to the more or less dangerous nature of the threat it offered to established religion. It is only as far as Athens and Imperial Rome are concerned that we have definite knowledge of the law and the judicial procedure on this point ...
In the criminal law of Athens we meet with the term asebeia --literally: impiety or disrespect towards the gods. As an established formula of accusation of asebeia existed, legislation must have dealt with the subject; but how it was defined we do not know. The word itself conveys the idea the law particularly had offences against public worship in view; and this is confirmed by the fact that a number of such offences--from the felling of sacred trees to the profanation of the Eleusinian Mysteries -- were treated as asebeia. When, in the next place, free-thinking began to assume forms which seemed dangerous to the religion of the State, theoretical denial of the gods was also included under asebeia.
6

The more practical outlook of the Romans may perhaps also have had something to say in the matter: they were rather indifferent to theoretical speculations, whereas they were not to be trifled with when their national institutions were concerned.
In consequence of this point of view the Roman government first came to deal with denial of the gods as a breach of law when confronted with the two monotheistic religions which invaded the Empire from the East. That which distinguished Jews and Christians from Pagans was not that they denied the existence of the Pagan gods--the Christians, at any rate, did not do this as a rule--but that they denied that they were gods, and therefore refused to worship them. They were practical, not theoretical deniers. The tolerance which the Roman government showed towards all foreign creeds and the result of which in imperial times was, practically speaking, freedom of religion over the whole Empire, could not be extended to the Jews and the Christians; for it was in the last resort based on reciprocity, on the fact that worship of the Egyptian or Persian gods did not exclude worship of the Roman ones. Every convert, on the other hand, won over to Judaism or Christianity was eo ipso an apostate from the Roman religion, an atheos according to the ancient conception. Hence, as soon as such religions began to spread, they constituted a serious danger to the established religion, and the Roman government intervened. ...
9-10

The Christians were generally designated as atheoi , as deniers of the gods, and the objection against them was precisely their denial of the Pagan gods, not their religion as such.
10

see also James Thrower, A short history of western atheism (Pemberton: London 1971)