2 John 9 read in the light of contemporary theological "progressivism"

"Anyone who is so 'progressive' [(Πᾶς ὁ προάγων)] as not to remain in the teaching of the Christ [(καὶ μὴ μένων ἐν τῇ διδαχῇ τοῦ Χριστοῦ)] does not have God; whoever remains in the teaching [(ὁ μένων ἐν τῇ διδαχῇ)] has the Father and the Son."

     2 John 9, NABRE.

2 John 9 OL Sabatier (1751)

     On the basis of 1) an Old Latin version of the passage (qui recedit, incl. departs from, abandons, separates or withdraws from, rather than the qui praecedit of the Vulgate, in Lucifer of Calaris), plus 2) three apparently regressive military uses of προάγω (in company with μένω) in Polybius (208-125 BC) and Diodorus Siculus (1st century BC), Terry Griffith, seeking to "put a brake on progressive interpretations running away with themselves", argues that "2 John 9a is . . . best translated in a more neutral way as 'Anyone who goes forth [or leaves] and . . . does not remain in the teaching of the Messiah'" (144):  "2 John 9. . . . need not reflect . . . a value judgment of 'progressiveness'" (142) considered "as a description of those holding to gnosticising or docetic trajectories of development" (144).  Terry Griffith, "The translation of Ο ΠΡΟΑΓΩΝ in 2 John 9," Tyndale bulletin 67, no. 1 (2016): 137–44, which, however, I find more cautionary than decisive.  I mean, my initial reaction to the first of his military examples in particular—But what if, in context (and having defeated Demetrius), Ptolemy had already started for Egypt, at least in intention?  Wouldn't the decision be, in that case, to proceed with the former plan of returning to Egypt?—is echoed later by Griffith himself when he says, "In these examples, the context alone determines whether the movement is viewed as a negative or a positive outcome, but the verb hardly means 'advance' and is probably better translated as 'proceed'" (143), in which case we would be back to a root sense without ruling out a metaphorical usage.
     Note, however, that the best critical edition of the Old Latin ed. Thiele (Epistulae Catholicae (Freiburg:  Verlag Herder, 1956-1969), 389-392), not cited by Griffith, presents a far more complex picture of the text than he does, with (though I have not mastered the apparatus) plenty of the Old Latin sources and church fathers opting for praecedit.
     Cf. Pope Francis to the German bishops:

The Pope further warns - with reference to a book by Pope Benedict XVI - against the 'temptation of the promoters of Gnosticism' who 'have always tried to say something new and different from what the Word of God has given them. ... What is meant by this is the one who wants to be ahead, the advanced one, who pretends to go beyond the "ecclesial We".' The passage from the Second Letter to John (2 John 9) mentioned in the text is revealing here: 'Any one who goes ahead, and does not abide in the doctrine of Christ, has not God'. The pope [adds] that there is 'a temptation by the Father of Lies …, who … ultimately dismembers the Body of the holy and faithful people of God'.

That from the "Schreiben von Papst Franziskus an das pilgernde Volk Gottes in Deutschland" 9, dated 29 June 2019, and composed originally in Spanish.  The reference is to Joseph Cardinal Ratzinger, Der Gott Jesus Christ:  Betrachtungen über den dreieinigen Gott =The God of Jesus Christ:  meditations on God in the Trinity, trans. Robert J. Cunningham (Chicago:  Franciscan Herald Press, 1979), 104-105, underscoring mine.

     John formulated quite knowingly these affirmations within the struggles of his own time as a sign that distinguished the Spirit from the anti-Spirit.  The great teachers of gnosis were interesting because of what they said in their own name, and they created a reputation for themselves.  They caused a sensation because they had something new and different to say that went beyond the Word.  For example, they claimed that Jesus in fact was not dead at all, but that he was dancing with his disciples while people believed him to be hanging on the cross.  The fourth Gospel opposed such gnostic novelties and some of their statements that were binding on on those who made them by using the ecclesiastical plural—this disappearance of the one who speaks behind the ecclesiastical 'we' really gives to the person who is speaking his true countenance and keeps it from dissolving into nothingness.  In John's epistles the same model was followed:  the author simply called himself 'the presbyter' [elder].  His adversary was the proagon, the one who advances (2 John 9).  The whole gospel of John as well as his epistles sought to be only a mobilization of memory, and in this respect it is the Gospel of the Holy Spirit.  To the extent that the Gospel does not invent something new but recalls its subject by meditating on it, it is fruitful, new, and deep.  The nature of the Holy Spirit, which is the unity of the Father and the Son, is forgetfulness of self (in which memory consists).  It is the true renewal.  A Church of the Spirit is a Church that, by remembering, penetrates more deeply into the Word, and thus becomes more alive and richer.  True forgetfulness of self and a detachment from self in order to reach Everything—this is a sign of the Spirit and a copy of his trinitarian nature.