Commentary on John, Book I, 23 (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.
Reproduced in full. Translation: Ante-Nicene Fathers, vol. 9 (Allan Menzies, 1896) (Public domain).
Why Pitre cites it
The oldest interpretation of Jesus' last words: no one understands John's Gospel 'who has not leaned on Jesus' breast nor received Mary from Jesus to be his mother also.' Pitre's closing witness that every disciple is invited to take Mary as mother.
Notes
Menzies: '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.' The key phrase ('leaned', 'received Mary from Jesus') is a different translation of the same sentence. 'first fruits of the Gospels' also occurs earlier in the same ANF paragraph ('I deem the first fruits of the Gospels to be ... the Gospel of John').