Today is the Solemnity of Mary, Mother of God, a feast that enshrines the doctrine of the Council of Ephesus. (For a bit of the dramatic story of that council, see my posts here and here.) Catholic Encyclopedia tells us…

Mary’s Divine motherhood is based on the teaching of the Gospels, on the writings of the Fathers, and on the express definition of the Church. St. Matthew (1:25) testifies that Mary “brought forth her first-born son” and that He was called Jesus. According to St. John (1:15) Jesus is the Word made flesh, the Word Who assumed human nature in the womb of Mary. As Mary was truly the mother of Jesus, and as Jesus was truly God from the first moment of His conception, Mary is truly the mother of God. Even the earliest Fathers did not hesitate to draw this conclusion as may be seen in the writings of St. Ignatius [Ephes 7], St. Irenaeus [Adv Haer 3.19], and Tertullian [Adv Prax 27]. The contention of Nestorius denying to Mary the title “Mother of God” [Serm 1.6.7] was followed by the teaching of the Council of Ephesus proclaiming Mary to be Theotokos in the true sense of the word. [Cf. Ambr., in Luc. II, 25, P.L., XV, 1521; St. Cyril of Alex., Apol. pro XII cap.; c. Julian., VIII; ep. ad Acac., 14; P.G., LXXVI, 320, 901; LXXVII, 97; John of Antioch, ep. ad Nestor., 4, P.G., LXXVII, 1456; Theodoret, haer. fab., IV, 2, P.G., LXXXIII, 436; St. Gregory Nazianzen, ep. ad Cledon., I, P.G., XXXVII, 177; Proclus, hom. de Matre Dei, P.G., LXV, 680; etc.]