He was the great Pope of the Council of Ephesus which took place the year before he died. This was the council which condemned Nestorius, the bishop and heretic who denied the Divine Maternity of the Blessed Virgin Mary. Nestorius claimed that there were two persons in Jesus, and Mary was the Mother of one, and God the Father in eternity the Father of the other. This is blasphemous heresy. There is only one Person in Jesus. That one Divine Person is both the Son of God the Father and the Child of Mary the Virgin. It was Saint Celestine, the Pope, who sent Saint Patrick to be the great apostle of Ireland in 432.