This theological- scientific topic has the main aim to consider the theological deeds of the Roman bishop Saint Celestine I (422–432 A. D.) and his personality in time in which Roman patriarchate was the pleader of the true Orthodox faith. Saint Celestine is a native Roman who performed great achievements in the time of his episcopal leadership at the Western Church: He officially criticized Nestorius’s heresy at the local council in Rome in 430, and owing to him the Christian evangelization of Ireland began by Saint Patrick’s mission. In the same time, he fought a strong theological battle against pelagianism which will be discussed in this study. We might say, that Pope Celestine I was the first Roman bishop who firmly stepped out against pelagianism and nestorianism. Taking into account the historical sources, we can be absolutely be sure that pelagianism and its followers were condemned in time of his pontificate regardless of the border of jurisdiction or ecclesiastical authorities. A very peaceful historical period marks the beginning of the reign of emperor Valentinianus III at the Western Church, but at the same time the Western Roman Empire began to decline. It is very important to point out Celestine’s spiritual fight against nestorianism in those difficult times when this heresy began to spread at the end of his pontificate. Nestorius speaks about the Nicene dogma of Incarnation in a way that incarnation is only the birth of „Jesus the man“ not the true Incarnation of God Logos as One Person of the Holy Trinity. This is the reason why Nestorius rejects the widespread term „Mother of God“ (Theotokos) and he does not want to accept the idea that God is incarnated from a Virgin, but only a man from Nazareth, because they are, according to Nestorius, two different persons. Saint Cyril of Alexandria reacted immediately and he wrote an Epistle to pope Celestine. Christology of St. Cyril of Alexandria was so precise in an Orthodox context, that it is clear that he teaches about the one undivided Person of the Christ who has both divine and human attributes. Pope Celestine considered Nestorius’s thought as a heretical danger to the true faith of the Eastern Church. The victory over Nestorius’s teaching was conciliarly declared on the Third Ecumenical Council in Ephesus in 431 A. D. where Saint Celestine sent his own delegates.