Four Discourses Against the Arians, III.29 — Nicene and Post-Nicene Fathers, series 2, vol. 4 (J. H. Newman, rev. Archibald Robertson, 1892) (public domain)
29. Now the scope and character of Holy Scripture, as we have often said, is this — it contains a double account of the Saviour; that He was ever God, and is the Son, being the Father's Word and Radiance and Wisdom; and that afterwards for us He took flesh of a Virgin, Mary Bearer of God, and was made man. And this scope is to be found throughout inspired Scripture, as the Lord Himself has said, 'Search the Scriptures, for they are they which testify of Me.' But lest I should exceed in writing, by bringing together all the passages on the subject, let it suffice to mention as a specimen, first John saying, 'In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by Him, and without Him was made not one thing;' next, 'And the Word was made flesh and dwelt among us, and we beheld His glory, the glory as of one Only-begotten from the Father;' and next Paul writing, 'Who being in the form of God, thought it not a prize to be equal with God, but emptied Himself, taking the form of a servant, being made in the likeness of men, and being found in fashion like a man, He humbled Himself, becoming obedient unto death, even the death of the Cross.' Any one, beginning with these passages and going through the whole of the Scripture upon the interpretation which they suggest, will perceive how in the beginning the Father said to Him, 'Let there be light,' and 'Let there be a firmament,' and 'Let us make man;' but in fullness of the ages, He sent Him into the world, not that He might judge the world, but that the world by Him might be saved, and how it is written 'Behold, the Virgin shall be with child, and shall bring forth a Son, and they shall call his Name Emmanuel, which, being interpreted, is God with us.'
Reproduced in full. Translation: Nicene and Post-Nicene Fathers, series 2, vol. 4 (J. H. Newman, rev. Archibald Robertson, 1892) (Public domain).
Why Pitre cites it
One of the first indisputable uses of 'Mother of God' (Theotokos), inside a treatise defending the divinity of Christ. Pitre stresses that the title arose from Christology, and that Athanasius ties it to Isaiah's Immanuel.
Notes
Newman/Robertson render Theotokos here as 'Mary Bearer of God' ('He took flesh of a Virgin, Mary Bearer of God, and was made man'); neither 'Mother of God' nor 'Theotokos' appears verbatim. §29 falls within NPNF chapter XXVI (§§26–41) of Discourse III.