This work examines the career of Metropolitan Petros (Astyfides) of Astoria (1915-1997), the founder of St. Markella’s Cathedral and first bishop of the Church of the Genuine Orthodox Christians of America (also known as the Greek Old Calendarist Church). Metropolitan Petros’s life spans the entire history of the Old Calendar movement and many of its major challenges, crises, and turning-points are reflected in his life. Therefore, he represents a microcosm of the movement, and study of his life sheds light on the Old Calendar movement in general.