In an essay on sacred law in ancient Athens (The Cambridge Companion to Ancient Greek Law, 67), Robert Parker assesses the scope and limits of freedom of speech and religion.
He notes, “If one asks how tolerant of unorthodox teachings (and cults) Athenian society was in practice, the answer will fluctuate depending on how one judges doubtful cases, but nothing would justify talk of systematic repression. If one asks whether the Athenians recognized an ideal of freedom of thought or of religious tolerance, the answer is unequivocally no: no text suggests that they did, and the decree of Diopeithes (if historical) and the successful charge against Socrates show them untroubled by such ideals. Freedom of speech a l’Athenienne meant the right of the poor man to make his voice heard alongside that of the rich, not a licence for impious talk.”
By the same token, private religious practices were allowed in Athens, but “if they came to be perceived as being socially undesirable, the undesirable behavior they encouraged could be rolled up along with ‘innovation in religion’ in a charge of impiety against their leader.” This wasn’t merely theoretical: “We know of three such prosecutions in the fourth century, all brought against women, two of which resulted in condemnation and execution.”