Why do we call the Virgin Mary “Theotokos”?

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A passage from the Letter of Saint Cyril of Alexandria to Nestorius.

“The holy and great Synod therefore says, that the only begotten Son, born according to nature of God the Father, very God of very God, Light of Light, by whom the Father made all things, came down, and was incarnate, and was made man, suffered, and rose again the third day, and ascended into heaven.

These words and these decrees we ought to follow, considering what is meant by the Word of God being incarnate and made man. For we do not say that the nature of the Word was changed and became flesh, or that it was converted into a whole man consisting of soul and body; but rather that the Word having personally united to himself flesh animated by a rational soul, did in an ineffable and inconceivable manner become man, and was called the Son of Man, not merely as willing or being pleased to be so called, neither on account of taking to himself a person, but because the two natures being brought together in a true union, there is of both one Christ and one Son; for the difference of the natures is not taken away by the union, but rather the divinity and the humanity make perfect for us the one Lord Jesus Christ by their ineffable and inexpressible union.

So then he who had an existence before all ages and was born of the Father, is said to have been born according to the flesh of a woman, not as though his divine nature received its beginning of existence in the holy Virgin, for it needed not any second generation after that of the Father (for it would be absurd and foolish to say that he who existed before all ages, coeternal with the Father, needed any second beginning of existence), but since, for us and for our salvation, he personally united to himself an human body, and came forth of a woman, he is in this way said to be born after the flesh; for the was not first born a common man of the holy Virgin, and then the Word came down and entered into him, but the union being made in the womb itself, he is said to endure a birth after the flesh, ascribing to himself the birth of his own flesh.

On this account we say that he suffered and rose again; not as if God the Word suffered in his own nature stripes, or the piercing of the nails, or any other wounds, for the Divine nature is incapable of suffering, inasmuch as it is incorporeal, but since that which had become his own body suffered in this way, lie is also said to suffer for us; for he who is in himself incapable of suffering was in a suffering body. In the same manner also we conceive respecting his dying; for the Word of God is by nature immortal and incorruptible, and life and life-giving; since, however, his own body did, as Paul says, by the grace of God taste death for every man, he himself is said to have suffered death for us, not as if he had any experience of death in his own nature (for it would be madness to say or think this), but because, as I have just said, his flesh tasted death. In like manner his flesh being raised again, it is spoken of as his resurrection, not as if tie had fallen into corruption (God forbid), but because his own body was raised again.

We, therefore, confess one Christ and Lord, not as worshipping. a man with the Word (lest this expression “with the Word” should suggest to the mind the idea of division), but worshipping him as one and the same, forasmuch as the body of the Word, with which he sits with the Father, is not separated from the Word himself, not as if two sons were sitting with him, but one by the union with the flesh. If, however, we reject the personal union as impossible or unbecoming, we fall into the error of speaking of two sons, for it will be necessary to distinguish, and to say, that he who was properly man was honoured with the appellation of Son, and that he who is properly the Word of God, has by nature both the name and the reality of Sonship.

We must not, therefore, divide the one Lord Jesus Christ into two Sons. Neither will it at all avail to a sound faith to hold, as some do, an union of persons; for the Scripture has not said that the Word united to himself the person of man, but that he was made flesh. This expression, however, “the Word was made flesh,” can mean nothing else but that he partook of flesh and blood like to us; he made our body his own, and came forth man from a woman, not casting off his existence as God, or his generation of God the Father, but even in taking to himself flesh remaining what he was.

This the declaration of the correct faith proclaims everywhere. This was the sentiment of the holy Fathers; therefore they ventured to call the holy Virgin, the Mother of God, not as if the nature of the Word or his divinity had its beginning from the holy Virgin, but because of her was born that holy body with a rational soul, to which the Word being personally united is said to be born according to the flesh.”

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