Greece is overwhelmingly Orthodox Christian, around 90 percent per most estimates including Pew Research's Religious Composition of the World, 2010-2020, with small Muslim, Catholic and religiously unaffiliated minorities; the Greek Orthodox Church remains constitutionally recognized as the country's prevailing religion, a formulation distinct from full establishment but granting it privileges including state salaries for clergy.
Ancient Greek religion, the polytheistic worship of Zeus, Athena, Apollo and the wider Olympian pantheon centered on sanctuaries such as Delphi and Olympia, gave way gradually to Christianity following the conversion of the Roman Empire from the fourth century CE, and Greece became a core territory of the Byzantine Empire, whose Orthodox Christianity, liturgy and theology it inherited directly after Constantinople's 1453 fall to the Ottomans left the Church as one of the few surviving institutions of Greek national identity during four centuries of Ottoman rule.
The Ecumenical Patriarchate of Constantinople, though physically located in modern Istanbul rather than Greece, remains the historic senior see of Orthodox Christianity and retains close ties to the Church of Greece, which gained formal autocephaly, administrative independence, in 1850. Greece's small Muslim minority, concentrated in Thrace near the Turkish border, is one of the few religious minorities specifically protected under an international treaty, the 1923 Treaty of Lausanne.