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Hellenism (Greek), the Hellenic ethnic religion ( ), also commonly known as Hellenismos, Hellenic Polytheism, Dodekatheism (?), or Olympianism (?), refers to various religious movements that revive or reconstruct ancient Greek religious practices, publicly, emerging since the 1990s.
Dodekatheism originated in and is practiced in Greece and in other countries. Leaders of the movement claimed in 2005 that there are as many as 2,000 adherents to the Hellenic tradition in Greece, with an additional 100,000 who have "some sort of interest". No official estimates exist for devotees worldwide.
There are no official naming practices for this religion, but there does seem to be an informal naming convention, based on academically accepted descriptive definitions, adhered to by groups and most individual believers. Hellenism is the most common term, used chiefly as an identifier for the modern polytheistic religion by its adherents today but it can also refer to the ancient Greek religion and culture. The term originally stems from a systematization and revival of Greek religion done by the Roman Emperor Julian. Julian used the term to describe traditional religion of the Greeks (the word can also have other unrelated meanings in modern Greek). Additionally, subgroups use a variety of names to distinguish branches focusing on specific schools of thought, or modern traditions focusing on the public practices of individual city-states. These subgroups can be described as denominations. Hellenic religion, and Hellenic polytheism can be said to be used interchangeably to refer to the religion, and are synonymous. The phrase Hellenic Polytheistic Reconstructionism refers more to the methodology used by some practitioners to revive a version of the religion, than the religion itself. Not all Hellenic Polytheists are reconstructionists. Dodekatheism and Olympianism are other names, though less commonly used.
The first Greek organization to openly support the religious revival of Hellenic religion was ? ? (Supreme Council of Ethnikoi Hellenes or YSEE), established in 1997, and is publicly active. YSEE is a founding member of the World Congress of Ethnic Religions (now European Congress of Ethnic Religions) and hosted the seventh annual WCER Congress in June 2004. YSEE is also a member of the European Union's action programme to combat discrimination. The organization primarily refers to the religion as the "Ethnic Polytheistic" or "genuine Hellenism" and its practitioners as Ethnikoi Hellenes, "Ethnic [National] Hellenes".
Another very active organization since its founding at 2008 is Labrys religious community. Labrys has focused primarily on the religious aspects of Hellenism or Hellenic (Greek) polytheism, avoiding anti-Christian rhetoric and politics, establishing weekly public rituals  and engaging in other aspects of practical promotion of polytheism like theater and music. Labrys has also promoted among Hellenes worldwide the need to actively practice household worship and the idea that family and community should be the starting points of religious practice. The community has been organizing since 2008 the largest festival in Athens and also actively participates and supports the religious aspects of the oldest Hellenic festival in Greece, Promitheia which is held every year on Mount Olympus. Labrys religious community has published the book about Hellenic Polytheism : Household Worship. Createspace. 2014-11-20. ISBN 9781503121881. .
Other Greek organizations, such as Dodekatheon (?, D?dekátheon, Of the Twelve Gods), the Helliniki Hetaireia Archaiophilon (Societas Hellenica Antiquariorum), and the Thyrsos use a combination of terms interchangeably, including (hell?nik? thr?skeîa, translated as "Hellenic religion"), Hellenic polytheistic religion, and Hellenism.
Dodecatheon and YSEE both use the terms "traditional", "ethnic", and "genuine" to refer to their religious practices. Greek polytheist author Vlassis Rassias has written a popular series of books on "Christian persecutions against the Hellenes," and the "Church of the Hellenes" organization goes so far as to call for the wholesale extermination of Christianity, while the Athens-based group Ellinais emphasizes "world peace and the brotherhood of man."
Outside Greece, Hellenic religious organizations began to emerge around 1998, with some individuals claiming to have been engaging in some form of traditional practice since the 1970s.
In the United States, the Hellenic polytheist organization Hellenion also identifies its practices as "Hellenic Pagan Reconstructionism" and emphasizes historical accuracy in its mission statement. The group uses the term "Hellenismos" (?, Hell?nismós) to describe the religion. Hellenion does not provide official membership numbers to the public, but an unofficial estimate of 43 members can be determined for 2007, though this number can only give the roughest approximation, as Hellenion offers hardship waivers to those who cannot afford the typical membership fees. In early 2010, the organization reported 1 demos (fully chartered local congregation) and 6 proto-demoi (start-up congregations not fully chartered with less than 3 members) established, which offer rituals and other events for members and frequently for the public as well. Two of the six proto-demoi cannot be independently verified to exist. Hellenion offers legal clergy training, basic adult religious education classes, and other educational/training courses for its members.
Another American group, Elaion, uses the term "Dodekatheism" (Greek: , dodeka, "twelve" + ?, theïsmós, "belief in the gods") to describe their approach to the Hellenic religion. No reported numbers for current membership levels are known to exist.
Other terms in common usage by Hellenic polytheists include "Greek reconstructionism" and "Hellenic Traditionalism", but the two are not synonymous.
In Brazil, in Portuguese language, there's the website of RHB - Reconstrucionismo Helênico no Brasil, built since 2003 by Brazilian members of Hellenion and other international groups, such as the American Neokoroi and the Greek Thyrsos.
Hellenic polytheists worship the ancient Greek Gods, including the Olympians, nature divinities, underworld deities (chthonic gods) and heroes. Both physical and spiritual ancestors are honored. It is primarily a devotional or votive religion, based on the exchange of gifts (offerings) for the gods' blessings. The ethical convictions of modern Hellenic polytheists are often inspired by ancient Greek virtues such as reciprocity, hospitality, self-control and moderation. The Delphic maxims, Tenets of Solon, the Golden Verses of Pythagoras, or even Aristotle's Ethics each function as complete moral codes that a Hellenic Polytheist may observe. Key to most ethical systems is the idea of kharis (or "charis", grace), to establish reciprocity between humanity and the gods, between individuals, and among community members. Another key value in Hellenic Polytheism is eusebeia, often translated as piety. This implies a commitment to the worship of the Hellenic gods and action to back this up.
There is no central "ecclesia" (church/assembly) or hierarchical clergy, though some groups (i.e., Hellenion) do offer training in that capacity. Individual worshipers are generally expected to perform their own rituals and learn about the religion and the gods by reference to primary and secondary sources on ancient Greek religion and through personal experience of the gods. Information gained from such personal experiences is often referred to in Hellenic groups as "UPG" (Unverified Personal Gnosis), a term borrowed from Ásatrú, though now commonly used among many pagan religions.
In polytheism, Reconstructionism is a methodology which attempts to accurately base modern religious practice on culturally and historically genuine examples of ancient religious practices. The term is frequently used in the United States to differentiate between syncretic and eclectic Neopagan movements, and those based on the traditions, writings, history, and mythology of a specific ancient polytheistic culture.
In contrast to the eclectic traditions, Reconstructionists are very culturally oriented and attempt to reconstruct historical forms of religion and spirituality, in a modern context. Therefore, Kemetic, Canaanite, Hellenic, Roman, Celtic, Germanic, Baltic and Slavic Reconstructionists aim for the revival of historical practices and beliefs of Ancient Egypt, Ancient Canaan and Phoenicia, Ancient Greece, Ancient Rome, the Celts, the Germanic peoples, the Balts and the Slavs, respectively.
Most Hellenic polytheist groups unequivocally state that reconstructionism is not the only correct method of practicing the ancient Greek religion, but do identify a practice as Hellenic only when it embraces the humanistic values and ethical virtues of the ancient Greeks, demonstrates loyalty and reverence toward the Greek gods, and uses a religious structure that would be recognizable to an ancient Greek. These groups make a clear distinction between themselves and the Neopagan movement, and identify some 'Hellenic' groups as "simply disguised as 'Hellenes' for reasons that exist hidden within the depths of their own minds." 
Revivalism focuses more on Hellenic Polytheism as a living, changing religion. Hellenic Revivalism allows more room for practitioners to decide what feels right to them. Most modern Hellenic Polytheists exist somewhere on a Reconstructionist to Revivalist spectrum.
Modern Hellenic polytheist organizations are "revivalist" or "reconstructionist" for the most part, but many adherents like Panagiotis Marinis from the group Dodecatheon in Greece, has stated that the religion of ancient Greece has survived throughout the intervening centuries, and that he, himself, was raised in a family that practiced this religion. Whether or not they believe that the Hellenic polytheist religious tradition is continuous, there is evidence that Greek Hellenic polytheists within the modern country of Greece see the movement as an expression of Greek cultural heritage, in opposition to the Orthodox Christianity that is overwhelmingly dominant.
The 2004 Summer Olympics stirred up several disputes concerning Hellenic polytheistic religion.