The Treatise of the Apostolic Tradition is a liturgical document of Rome dating from the year 215 approximately, attributed to St Hippolytus of Rome. It mentions the Virgin Mother of Christ, the Word of God, Savior of man:
A) At the heart of the Eucharistic celebration, at the moment when the celebrant gives thanks – a brief and essentially Christological action – the Virgin Mother is mentioned twice:
“We give you thanks, O God, for your beloved son Jesus Christ, whom you sent to us in former times as Savior, Redeemer, and Messenger of your Will, who is your inseparable Word,
through whom you made all, and in whom you were well-pleased, whom you sent from heaven into the womb of a virgin, who, being conceived within her, was made flesh, and appeared as your Son, born of the Holy Spirit and the Virgin.”
(The Anaphora or Eucharistic Prayer of the Roman Rite , Tradition Apostolique 4, from the work of B. Botte: La Tradition Apostolique. Sources Chretiennes, 11 bis. Paris, Cerf, 1968, p. 49)
B) In the rite of baptism for the Easter Vigil, we have the oldest example of an "Apostolic Symbol." At the second immersion, the catechumen is asked:
“Do you believe in Jesus Christ, the Son of God, who was born of the Holy Spirit and the Virgin Mary?”
(Apostolic Tradition 21, French text by B.BOTTE, SC 11 bis, Cerf 1968, p. 84)
These documents are important; in them the 3rd-century Church professes and celebrates her own faith. In them we also find the memory of the virginal conception and the birth of Christ from the Holy Spirit and the Virgin.
Mary, true mother and virgin mother, is part of the dogmatic doctrine of the Church in two fundamental moments of ecclesial life: the Eucharistic meal and baptism.
The motive is obvious: Mary’s true motherhood to Christ and her virginal fecundity are the historic base and the guarantee of salvation. "Signum virginis", as Saint Irenaeus had taught.
This explains why only the Virgin Mother is named, and not the angels, patriarchs, prophets, apostles and martyrs: this is a doctrinal commemoration of the mystery of the incarnation where Mary played a unique and essential function.
The Church of the 3rd century incorporates and fixes in a formula of faith the doctrine of the birth of Christ, Verb of God, Savior of man: "Of the Holy Spirit and the Virgin," (Apostolic Tradition 4) and again " Of the Holy Spirit and the Virgin Mary" (Apostolic Tradition 21).
This formula was inserted definitely in the Symbol of the Constantinople Council, universally adopted in all the liturgies and explained in all its salvific vigor: "By the power of the Holy Spirit he was born of the Virgin Mary, and became man."
"For a child is born to us, and a son is given to us, whose government is upon his shoulder: and his name is called the Messenger of great counsel: for I will bring peace upon the princes, and health to him. His government shall be great, and of his peace there is no end." (Is 9:6)