Religious Rituals, Festivals, Sacrifices
Ancient Greek Mythology, Religion, Art
Although it may be common to speak of a Greek "religion," in fact the Greeks themselves didn't use such a term and might not have recognized it had someone else attempted to apply it to their practices. It's difficult to accept the idea that the Greeks where completely secular and irreligious, however. This is why a better understanding of Greek religion helps illuminate the nature of religion generally as well as the nature of religions which continue to be followed today. This, in turn, is critical for anyone who wants to engage in a sustained critique of religion and religious beliefs.
If we mean by "religion" a set of beliefs and behavior which are consciously chosen and ritually followed to the exclusion of all other alternatives, then the Greeks didn't really have a religion. If, however, we mean by religion more generally people's ritual behavior and beliefs about sacred items, places, and beings, then the Greeks most certainly did have a religion - or perhaps a set of religions, in recognition of the great variety of Greek beliefs.
This situation, which appears odd to most modern eyes, forces us to reconsider what it means to talk about a "religion" and what is essentially "religious" about modern religions like Christianity and Islam. Perhaps when discussing Christianity and Islam as religions, we should look more closely at the beliefs about what is sacred and holy and less at their exclusiveness (this is precisely what some scholars, like Mircea Eliade, have argued).
Then again, perhaps their exclusiveness is precisely what deserves the most attention and critique because this separates them from ancient religions. Whereas the Greeks seemed quite willing to accept foreign religious beliefs - even to the point of incorporating them into their own cosmology - modern religions like Christianity tend to be highly intolerant of innovations and new additions. Atheists are labeled "intolerant" for daring to criticize Christianity, but can you imagine Christian churches incorporating Muslim practices and scriptures in the way that Greeks incorporated foreign heroes and gods into their own rituals and stories?
Despite their variety of beliefs and rituals, though, it is possible to identify a set of beliefs and practices that distinguish the Greeks from others, allowing us to talk at least a bit about a coherent and identifiable system. We can discuss, for example, what they did and did not regard as sacred then compare this against what is considered sacred by religions today. This, in turn, may help chart the development of religion and culture not just in the ancient world, but also the ways in which those ancient religious beliefs continue to be reflected in modern religions.
Classical Greek mythology and religion did not spring fully formed from the rocky Greek ground. They were, instead, amalgams of religious influences from Minoan Crete, Asia Minor, and native beliefs. Just as modern Christianity and Judaism have been significantly influenced by ancient Greek religion, the Greeks themselves were heavily influenced by the cultures that came before. What this means is that aspects of contemporary religious beliefs are ultimately dependent upon ancient cultures that we no longer have any access to or knowledge of. This differs sharply from the popular idea that current religions were created by divine command and without any preceding basis in human culture.
The development of a recognizably Greek religion is characterized in large part by conflict and community. Greek mythological stories which everyone is familiar with are defined to a great extent by conflicting forces while Greek religion itself is defined by attempts to reinforce a common sense of purpose, civic cohesion, and community. We can find very similar concerns in modern religions and in the stories which Christians today tell each other - though in this case this is likely due to how these are issues which are community to humanity as a whole rather than through any direct cultural influence.
Hero cults, both in ancient Greece as well as contemporary religions, tend to be very civic and political in nature. Their religious elements are certainly undeniable, but religious systems typically serve the political community - and in ancient Greece, this was true to a greater degree than one usually sees. Veneration of a hero bound the community together around a glorious past and it was here that the roots of families and cities could be identified.
Similarly, many Americans today see their nation as rooted in the deeds and promises attributed to Jesus in the New Testament. This technically contradicts Christian theology because Christianity is supposed to be a universal religion in which national and ethnic distinctions are supposed to disappear. If we see ancient Greek religion as representative of some of the social functions which religion was created to serve, though, then the behavior and attitudes of Christians in America begin to make sense because they simply stand in a long line of using religion for the purpose of political, national, and ethnic identity.