"The Virgin Mary, being obedient to his word, received from an angel the glad tidings that she would bear God" (Against Heresies, 5:19:1 [A.D. 189]).
"[T]o all generations they [the prophets] have pictured forth the grandest subjects for contemplation and for action. Thus, too, they preached of the advent of God in the flesh to the world, his advent by the spotless and God-bearing (theotokos) Mary in the way of birth and growth, and the manner of his life and conversation with men, and his manifestation by baptism, and the new birth that was to be to all men, and the regeneration by the laver [of baptism]" (Discourse on the End of the World 1 [A.D. 217]).
"Though still a virgin she carried a child in her womb, and the handmaid and work of his wisdom became the Mother of God" (Songs of Praise 1:20 [A.D. 351]).
"The Word begotten of the Father from on high, inexpressibly, inexplicably, incomprehensibly, and eternally, is he that is born in time here below of the Virgin Mary, the Mother of God" (The Incarnation of the Word of God 8 [A.D. 365]).
"We confess, then, our Lord Jesus Christ, the only begotten Son of God, perfect God and perfect man, of a rational soul and a body, begotten before all ages from the Father in his Godhead, the same in the last days, for us and for our salvation, born of Mary the Virgin according to his humanity, one and the same consubstantial with the Father in Godhead and consubstantial with us in humanity, for a union of two natures took place. Therefore we confess one Christ, one Son, one Lord. According to this understanding of the unconfused union, we confess the holy Virgin to be the Mother of God because God the Word took flesh and became man and from his very conception united to himself the temple he took from her" (Formula of Union [A.D. 431]).