One of the Doctors of the Church, Saint Cyril (d. 444) is known for countering the heresy by Nestorius, which claimed that the Incarnate Christ was not fully and simultaneously God and man.

At the Council of Ephesus in 431, Cyril asserted that there could be no atoning of sin if Christ was not man, and that Our Lord must also be God, in order to redeem that which he had become.

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