Catechism of the Catholic Church §§466, 495 — English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va (official text, quoted at the cited paragraph)
Catechism of the Catholic Church §466 The Nestorian heresy regarded Christ as a human person joined to the divine person of God's Son. Opposing this heresy, St. Cyril of Alexandria and the third ecumenical council, at Ephesus in 431, confessed "that the Word, uniting to himself in his person the flesh animated by a rational soul, became man." Christ's humanity has no other subject than the divine person of the Son of God, who assumed it and made it his own, from his conception. For this reason the Council of Ephesus proclaimed in 431 that Mary truly became the Mother of God by the human conception of the Son of God in her womb: "Mother of God, not that the nature of the Word or his divinity received the beginning of its existence from the holy Virgin, but that, since the holy body, animated by a rational soul, which the Word of God united to himself according to the hypostasis, was born from her, the Word is said to be born according to the flesh."
Catechism of the Catholic Church §495 Called in the Gospels "the mother of Jesus", Mary is acclaimed by Elizabeth, at the prompting of the Spirit and even before the birth of her son, as "the mother of my Lord". In fact, the One whom she conceived as man by the Holy Spirit, who truly became her Son according to the flesh, was none other than the Father's eternal Son, the second person of the Holy Trinity. Hence the Church confesses that Mary is truly "Mother of God" (Theotokos).
This text is under copyright; the cited paragraph is quoted for study, with a link to the full document. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Why Pitre cites it
The Council of Ephesus and the title Theotokos, Mother of God — the Catechism's summary of the doctrine Pitre traces to Luke 1:43 and Matthew 1:23.
Notes
Paragraph names the Council of Ephesus (431) and quotes its confession of Mary as "Mother of God"; the Greek word Theotokos itself does not appear in §466 (it appears in §495). Footnote numbers 89–90 removed.