The national religion of Greece is the Greek Orthodox Church. It dates back to 105 ad when the Christian Church divided into Roman Catholic and Eastern Orthodox Churches (Orthodox meaning "right belief").

According to Greek Orthodox tradition, every day of the year is dedicated to a Christian saint or martyr. January 7 for example, is the day of Saint John the Baptist and it is the name day for all Greek people named Yannis (or Ioannis = John, male) or Yanna (or Ioanna, female). A person's Name Day is considered more important than the same person's actual birthday and it is always celebrated in Greece.

Hydra Island is dominated by the Greek Orthodox Church. Its influence is found everywhere on the island. The Cathedral, with it's magnificent bell tower, dominates the port while simpler chapels and churches are scattered on almost every corner.