Hellenismos, a.k.a. Hellenism, Dodekatheism, Olympianism, etc., are terms describing the modern day revival and worship of the traditional Greek gods. The term Hellenismos was coined by Roman Emperor Julian, the last pagan emperor of the Roman Empire.
Hellenismos officially arose in the 1990s, with the creation of YSEE, the Supreme Council of Ethnikoi Hellenes, though worship and veneration of the Greek gods has occurred sporadically over the centuries after the coming of Christianity. Since 1997 multiple groups and organizations have come into existence. Some of these groups seek peace and tolerance for all religions while others call for the extinction of Christianity over it's long history of religious persecution. Similarly, some organizations are politicized and require their members to be Greeks while others accept any member, regardless of heritage or nationality.
Hellenisimos differs from Hellenic Reconstructionism in the sense that members of Hellenismos are primarily concerned with the worship of the gods and not the complete revival of ancient Greek life. Hellenismos also differs from Wicca and NeoPaganism in multiple ways, mainly through it's strict adherence to the belief that the Greek gods are separate from the gods of other religions, it's rejection of practicing magic or witchcraft, the method of worship, and it's rejection of invocations of the gods, the concept of personal patron gods, as well as viewing the gods as if they are equals with humans.
Worship in Hellenismos is primarily focused on the twelve Olympians;
Zeus - The king of the gods, god of the sky and weather
Hera - The queen of the gods, goddess of marriage, maternity and women
Poseidon - The god of the seas and oceans
Demeter - The goddess of agriculture
Athena - The goddess of wisdom, justified war, and the arts and crafts
Apollo - The god of civilization, rationality, light, prophecy, music, healing, and art
Artemis - The goddess of hunting, animals and wild nature
Ares - The god of unjustified war or "war for war's sake".
Aphrodite - The goddess of love, beauty, sex and fertility
Hephaestus - The god of blacksmithing, metallurgy, fire and volcanoes
Hermes - The messenger of the gods, god of commerce, trade, and communication
Hestia - The goddess of the hearth and home
The list of the 12 Olympians varies among sources, even from ancient times. In some versions Hestia is replaced with Dionysus, the god of wine, freedom and ecstasy. A very popular belief is that Hestia willingly gave up her throne on Olympus for Dionysus as she would rather sit and tend to the hearth instead. In practical purposes, the 12 great Olympians are those gods who sit on the 12 thrones on Olympus. In the wider view, any god who dwells on Olympus can be called an Olympian. In addition to the 12 great gods listed above, as well as Dionysus, there are a multitude of other gods who were believed to either permanently or temporarily reside on Olympus. These other gods can be classified as Olympians.
Other than Olympians, members of Hellenismos also worship the following categories of deities:
The Primordial Gods - The gods of the elements that arose during creation.
The Titans - The offspring of earth and sky, the first race of gods
The Rustic Gods - The gods of the earth who do not reside on Olympus
The Marine Gods - The gods of the oceans and seas that do not reside on Olympus
The Chthonic Gods - The gods of the underworld that do not reside on Olympus
The Daemons - Spirits in-between man and the gods
The Nymphs and Satyrs - Minor gods of nature
Heroes - Humans, mostly demigods, who achieved fame and recognition through their acts
The Ancestors - The spirits of deceased loved ones
*Note: There are both personal and impersonal deities. The personal deities are those that are conscious and are believed to hear and answer prayers. The impersonal deities are those deities that may or may not be conscious but do not hear or answer prayers. Many practitioners of Hellenismos do not worship the impersonal gods though they may refer to them out of respect.
Methods of worshiping the deities primarily include prayer, praise, the recitation or singing of hymns and poetry, libations, the burning of incense, and the making of offerings. Unlike in the past, animal sacrifice is no longer considered acceptable for the gods. Many practitioners of Hellenismos point to the teachings of Orpheus and Greek philosophers who taught that the gods do not require the taking of life or the spilling of blood. Instead of animal sacrifice, offerings of water, milk, honey, wine, beer, fruit, vegetables and bread are made. Additionally, candles, incense, gifts and poems may be given as offerings. Most worship is done in the home or in an outdoor setting. In the future it may very well be that public temples will be built where the gods can be worshiped openly. With regards to prayer, Hellenismos differs greatly with other religions. It is a common belief among practitioners of Hellenismos that prayer should only be reserved for serious issues and should not be done for trivial matters. The term "God helps those who help themselves" is quite apt. Interestingly, the phrase is not Christian in origin and in fact originated with the ancient Greeks. So "The gods help those who help themselves" is a much better translation.
The rites and rituals of Hellenismos are quite similar in function to most religions. There are rites for the great stages of human life, birth, adulthood, marriage, and death. There are rites performed at specific festivals honoring one or more deities held throughout the year. The rites can be elaborate or simple, depending upon the ideals of the worshiper.
Hellenismos has no formal belief in the concept of sin as found in other religions. However, Hellenismos does teach the importance of being spiritually clean as well as avoiding hubris. One of the requirements of Hellenismos is that practitioners must be clean when approaching the gods. This can be accomplished through regular or ritual bathing, if only the cleaning of the hands and feet. Hubris is a difficult word to translate into English. In a nutshell, hubris is man/woman acting arrogantly, with excessive pride and with the implication that he/she is equal to or greater than the gods. It is strongly believed that to act with hubris is to invite disaster and tragedy.
When it comes to belief in the afterlife, practitioners of Hellenismos are free to form their own opinions and beliefs. The traditional belief of the ancient Greeks was that there was no afterlife, that death was the end. The mythology of the soul traveling to Hades was in fact just a metaphor of the grave. With the coming of the Mystery Religions, belief in a place of reward for good people and punishment for evil people began to become popular. The philosophers also interjected a belief in reincarnation. So one is perfectly free to believe what one wishes with regards to what comes after death.
As far as mythology goes, most members of Hellenismos stress that the myths were created by humans and reflect the ideals of the time period in which they were written. Therefore, the myths of the gods are not viewed as inerrant or unchangeable. Any myth that possesses ideals or concepts that modern people may find offensive can be edited or rewritten to reflect the values of modern society.
Though a minority religion at present, Hellenismos is believed to become one of the fastest growing religions of the future. Greek mythology captures the imagination of millions of children each year and it's inevitable that a portion of such children grow up to become adults who make the conscious choice to honor the old ways.