There are four christologically charged statements in 1 John that are presented as relevant to the apostasy: (1) "Who is the liar but the one who denies that Jesus is the Christ/Messiah," 2:22; (2) "Every spirit that confesses that Jesus Christ has come in the flesh," 4:2; (3) Every spirit that does not confess Jesus is not from God," 4:3; (4) "God abides in those who confess that Jesus is the Son of God," 4:15. The first of these (2:22) could be interpreted as referring to the separationist christology of Cerenthus, that Jesus and Christ were two separate entities; it also could refer to the denial of Jesus being the Messiah or the promised Christ. However (2) does not address Cerenthian christology because it does posit Jesus as the fleshly vessel of the Christ, and is thus thought to apply to the kind of docetism attacked by Ignatius and Polycarp. Griffith's point is that this too could simply refer to a denial that the Christ/Messiah has come. He says that his proposal is attractive in terms of simplicity because one would not need to posit two separate heresies that the author was attacking.
In support of the messianic interpretation of 2:22, Griffith points out that the term Messias in all the NT occurs only in John 1:41 and 4:25, where it is specified as the equivalent of Khristos (thus it was not a foreign concept to the Johannine community), and he also says that in 1 John 2:22 ho Khristos appears to be titular (that is, not a personal name) and the subject of the einai clause since it contains an article whereas Iesous does not (e.g. "that the Christ/Messiah is Jesus"). The big weakness I think of this is that it unnecessarily attributes too much of the "Messiah" notion to the Asia Minor Johannine community which otherwise did not develop it in their high christology. But I think I might agree that Christ more naturally is titular and that the passage more likely refers to those as rejecting Jesus as being (in the role of) Christ, whatever the title Christ might mean in the Johannine community. The arguments against the docetic interpretation of 4:2 seem to be a bit less tenuous. Griffith points out that the phrase "come in the flesh" or "appeared/manifested in the flesh" quite commonly refers to the fact of the incarnation and not its manner (e.g. non-docetic incarnation) in early Christian writings:
"But [Jesus] himself submitted, in order that he might destroy death and demonstrate the reality of the resurrection of the dead, because it was necessary that he be manifested in the flesh (en sarki edei auton phanerothenai). Also, he submitted in order that he might redeem the promise of the fathers and while preparing the new people for himself prove, while he was still on earth (epi tes ges on), that after he has brought about the resurrection he will execute judgment.... For if [Jesus] had not come in the flesh (me elthen en sarki), men could in no way have been saved by looking at him. For when they look at merely the sun they are not able to gaze at its rays, even though it is the work of his hands will eventually cease to exist. Therefore the Son of God came in the flesh (en sarki elthen) for this reason, that he might complete the full measure of the sins of those who persecuted his prophets to death" (Barnabas 5:6-7, 10-11).
Here "coming in the flesh" refers to Christ's entrance into the human sphere and his work therein (cf. especially the phrase epi tes ges "while on earth"); there is no "christological axe to grind here", the focus is soteriological. Barnabas even uses this phrase "in the flesh" to criticize Jews who reject Jesus as the Christ (e.g. Messiah), just the usage that Griffith posits for 1 John:
"Observe again that it is Jesus, not a son of man but the Son of God, and revealed in the flesh (en sarki phanerotheis) by a symbol. Since, however, they were going to say that the Messiah (Khristos) is the Son of David, David himself, fearing and understanding the error of sinners, prophesied: 'The Lord said to my Lord, "Sit at my right hand until I make your enemies a footstool for your feet" ' " (Barnabas 12:10).
Quite far from being anti-docetic, the above passage appears to even question a human paternity of Jesus as a "son of man" and "the Son of David" (the latter being a mainstay of Jewish Messianic thought). It also uses Khristos in a titular sense, as the Christ that the Jews expect. A use of epi ges "on earth" in connection with the phrase "in the flesh", similar to Barnabas 5:6-11 above, also appears in a Christian interpolation into the Testament of Benjamin, which also criticizes Jews who rejected the Messiah:
"The king of heaven appeared on earth (ton epi ges phanenta) in the form of a man of humility.... And the Lord will judge Israel first for the unrighteousness done to him, because they did not believe that God appeared in the flesh (paragenamenon theon en sarki) as a deliverer. And then he will judge all the Gentiles as many as did not believe him when he appeared on the earth (epi ges phanenti)" (Testament of Benjamin 10:7-8)
The proto-gnostic Gospel of Thomas also has Jesus declaring: "I took my place in the midst of the world (en meso tou kosmou), and I appeared to them in the flesh (en sarki ophthen)" (28:1). One final example of "in the flesh" being paralleled with "on the earth" appears in the Epistle to Diognetus:
"They [Christians] are in the flesh (en sarki), but they do not live according to the flesh. They live on the earth (epi ges), but their citizenship is in heaven.... Christians dwell in the world (en kosmo), but are not of the world" (Diognetus 5:8-9, 6:3)
The frequent connection between an appearance or manifestation "in the flesh" and the same "on the earth" shows that the former signifies one's presence in the earthly sphere and not the corporeality of one's body. 1 Timothy 3:16 also appears to use the phrase "appeared in the flesh" as shorthand to refer to Christ's earthly life altogether. Similarly, it is used to refer to the apostles' own earthly lives in Galatians 2:20; Philippians 1:22, 24; 1 Peter 4:2. Ignatius, Ephesians 1:3 refers to Onesimus as "your bishop in the flesh (en sarki episkopo)", and a similar use of en sarki occurs in 2 Clement 8:2 where it is paralleled with en kosmo "in the world". Finally, we read in the Protevangelium of James that "it has been revealed by the Holy Spirit that [Simeon] should not see death (me idein thanatou) until he had seen in the flesh (en sarki), the Christ" (24:4). The contrast again clearly shows that "in the flesh" is an idiom for one's earthly life.
In view of the use of the phrase in proto-gnostic writings (i.e. Gospel of Thomas) and writings alluding directly to the descent of God in human form (e.g. Testament of Benjamin), its use in polemic that has nothing to do with high christology (e.g. Barnabas), and it's existence as an idiom simply denoting one's earthly life, the expression "come in the flesh" doesn't sound specifically anti-docetic to me. A more clearly anti-docetic expression is the use of eis "into" instead of en "in" (i.e. entry into flesh), and this formation is what is used in the clearly anti-docetic 3 Corinthians:
"What [Simon and Cleobius] say and teach is as follows ... that the Lord has not come into the flesh (eis sarka elthen ho kurios).... our Lord Jesus Christ was born of Mary of the seed of David, the Holy Spirit being sent forth from heaven from the Father into her, that he might come down into this world and redeem all flesh by his flesh, and raise us up from the dead in the flesh.... For by his own body Jesus Christ saved all flesh, that he might show forth the temple of righteousness in his body.... Since therefore the Lord has had mercy on us, that while you are still in the flesh (en sarkei) we may hear these things again from you" (3 Corinthians 1:6, 14; 3:16-17)
This text is overtly anti-heretical, emphasizes repeatedly the role of Jesus' flesh and "his own body" in the redemption and uses a more precise expression with eis "into" for the incarnation. But interestingly, it still uses "in the flesh" in 1:6 to refer to the existence of Christians in the earthly sphere. One of the few anti-docetic texts that clearly does use en sarki "in the flesh" as an anti-docetic device is Ignatius, Smyrnaeans 3:1 where he says "I know and believe that he was in the flesh (en sarki) even after the resurrection". But the passage in which this phrase occurs is also studded with many anti-docetic devices that leave no doubt as to what is meant, including the repeated use of altheos "truly", the specific word dokein "appearance", the phrase en anthropo theos "God in man", the compound sarkophoren "bearing flesh", and even a logion from Jesus directly refuting the concept. If we go back to 1 John, is there any other similar independent evidence aside from the phrase en sarki that docetism is specifically meant? Griffith suggests that "Jesus Christ come in the flesh" is one of several expressions in 1 John that refer to Christ's former existence in the earthly sphere: (1) "the Son of God was revealed" (3:8; cf. 1:2; 3:5); (2) "God sent his only Son into the world" (4:9; cf. 4:14); (3) "the Son of God has come" (5:20). By extension, the naysayers would be those denying that the Son of God was revealed, sent into the world, and had come. Griffith also observes two more things about 1 John 4:2-3. First, 4:2 embeds the reference to the flesh within a participle modifying "Jesus Christ" and not as a clause in its own right with its own emphasis. In other words, we should read "every spirit that confesses Jesus-Christ-came-in-the-flesh is from God". If the polemic was targeted against docetism, we would better expect an infinitive clause, e.g. "every spirit that confesses that Jesus Christ came (inf.) in the flesh". In fact, the Codex Vaticanus (alone of the codices) admended the text specifically with the infinitive eleluthenai "to have come" to achieve a more anti-docetic reading, and curiously the same form occurs in the quotation in Polycarp (Philippians 7:1) as well. Second, the anti-confession in 1 John 4:3 specifically omits the words en sarki eleluthota "came in the flesh", suggesting that what matters is confession of Jesus (cf. "God abides in those who confess that Jesus is the Son of God," 4:15) and not the mode of his incarnation.
Those are most of Griffith's arguments, but he goes into more detail on the allusions to "idolatry" in the epistle and how they may relate to Judaizing apostasy.