Mary & the Saints
Mary as Mother of God
Because Jesus is one divine Person with two natures, Mary who bore him is rightly called Theotokos, the Mother of God.
Catholic teaching
The title 'Mother of God' safeguards the truth of the Incarnation: Mary did not merely give birth to a human nature but to the eternal Son of God who took flesh from her. Denying the title, as Nestorius did, implicitly divides Christ into two persons, a Christological error condemned by the Church.
Biblical foundation
Elizabeth, filled with the Holy Spirit, greets Mary as 'the mother of my Lord,' applying to her the divine title 'Lord' reserved for God. John's prologue teaches that the Word who was God 'became flesh,' and it is this same Word whom Mary conceived and bore.
Scripture references
Historical evidence
The Council of Ephesus in AD 431 formally defined Mary as Theotokos against Nestorius, and the title was already in devotional use, as attested by the ancient Egyptian prayer 'Sub tuum praesidium,' dated by papyrologists to as early as the third century.
The Church Fathers
“We do not say that his flesh was changed into the nature of divinity... we say the Word... is called Son of Man, and mother of God the Holy Virgin.”