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Pope Celestine I served as pope in the 5th century, specifically from 422 to 432. His papacy was marked by several significant events, including the Third Ecumenical Council of Ephesus in 431, which addressed the Nestorian heresy. Celestine also played a pivotal role in strengthening the authority of the papacy and establishing papal supremacy over other bishops, solidifying the foundation for the future development of the Catholic Church.