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The Greek Orthodox Church dominates life in Hydra as it does throughout Greece. On Hydra there are so many churches and chapels that you could visit a different one every day of the year and not see them all.
The Orthodox religion is Christian, which was practiced along the same lines as Catholicism until it split from the Catholic Church around the 14th Century. The most obvious difference since the religious split is that the Orthodox religion follows a different dating system. Hence the Orthodox and Catholic religions have different dates for Pascha and Easter. During the Ottoman era, Hydra (as well as all occupied Greece) continued to practice their Greek Orthodox Religion alongside the Ottoman Muslim Religion.