Commentary on John, Book I, 4 (ANF Book I, ch. 6) — Ante-Nicene Fathers, vol. 9 (Allan Menzies, 1896) (public domain)
We may therefore make bold to say that the Gospels are the first fruits of all the Scriptures, but that of the Gospels that of John is the first fruits. No one can apprehend the meaning of it except he have lain on Jesus' breast and received from Jesus Mary to be his mother also. Such an one must he become who is to be another John, and to have shown to him, like John, by Jesus Himself Jesus as He is. For if Mary, as those declare who with sound mind extol her, had no other son but Jesus, and yet Jesus says to His mother, “Woman, behold your son,” and not “Behold you have this son also,” then He virtually said to her, “Lo, this is Jesus, whom you bore.” Is it not the case that every one who is perfect lives himself no longer, but Christ lives in him; and if Christ lives in him, then it is said of him to Mary, “Behold your son Christ.”
Reproduced in full. Translation: Ante-Nicene Fathers, vol. 9 (Allan Menzies, 1896) (Public domain).
Why Pitre cites it
'There is no child of Mary except Jesus, according to the opinion of those who think correctly about her' — the perpetual virginity as a touchstone of right thinking, in the third century.
Notes
ANF 9 numbers this Book I, ch. 6 ('The Fourfold Gospel. John's the First Fruits of the Four...'); modern editions cite it as I.4.23. Menzies' wording: 'For if Mary, as those declare who with sound mind extol her, had no other son but Jesus' — not the key phrase verbatim. The span opens with the first-fruits sentences (overlapping item origen-john-1.23) because the Mary sentence begins 'For if' and depends on them.