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Feodor I, the "Great Synaxaristes," was a renowned 16th-century scholar and compiler in the Orthodox Church. His most notable contribution is the extensive work known as the "Great Synaxarion," a comprehensive collection of biographies and accounts of saints and martyrs revered in the Orthodox tradition. Feodor I meticulously documented their lives, miracles, and feast days, creating an invaluable resource for religious study, veneration, and spiritual inspiration.