I am a Christian of the Protestant persuasion, and present a few extracts from John Gill’s Exposition of the Bible on 2 John 1:9. John Gill (1697-1771), a Baptist minister, held to a firm Calvinistic soteriology. https://en.wikipedia.org/wiki/John_Gill_(theologian)
Extracts from John Gill’s Exposition of the Bible on 2 John 1:9:
Whosoever transgresseth: who passes over the rule and standard of doctrine, the word of God, and will not adhere to that, nor walk according to it, but rejects and despises that rule:
and abideth not in the doctrine of Christ; which he received from his Father, and delivered to his apostles, and of which he is the sum and substance; the doctrine which is concerning his person as the Son of God, and as truly God, and the union of the two natures, divine and human, in his one person; and concerning his office, as the Mediator, surety, and messenger of the covenant, and as the prophet, priest, and King of his church; and concerning his incarnation, obedience, sufferings, death, resurrection from the dead, ascension to heaven, session at God's right hand, intercession for his people, and second coming to judgment; concerning peace and pardon by his blood, atonement by his sacrifice, justification by his righteousness, and complete salvation by him: this is (xyvmh Klm lv wdwmlt) , "the doctrine of the King Messiah", or the Messiah's Talmud F8, to use the Jewish phrase, and which agrees with John's. Now, whoever has embraced and professed this doctrine, but errs concerning it, and rejects it, and abides not in it, as Satan abode not in the truth, appears to be of him:
He that abideth in the doctrine of Christ; as he hath received and professed it; neither can anything remove him from it, not the arguments of false teachers, nor the reproaches and persecutions of men, or the snares and allurements of the world:
The New Living Translation Study Bible expresses 2 John 1:9 this way:
Anyone who wanders away from this teaching has no relationship with God. But anyone who remains in the teaching of Christ has a relationship with both the Father and the Son. (2 John 1:9 NLT)
The NLT Study Bible makes this comment regarding 2 John 1:9:
Literally, everyone going beyond and not remaining in the teaching. To go beyond is to contradict the apostolic teachings about Jesus Christ and believe things about Jesus that were not taught by the apostles.
At the time the apostle John wrote this second letter, Docetists were infiltrating the early church and denying that Jesus Christ came in a real body. The word Docetism comes from the Greek dokein, which meant “to seem.” According to Docetism, Jesus Christ only seemed to have a human body. Docetism was closely related to Gnosticism, which viewed physical matter as inherently evil and spiritual substance as inherently good. The apostle John warned the early church against the false doctrine of Gnosticism, which embraced Docetism’s error: “This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist.” (1 John 4:1–2 NLT)
Early church fathers fought valiantly against Docetism, especially Ignatius of Antioch (c. AD 35–107). Ignatius rightly taught that, if Jesus had not actually shed His blood on the cross, then His death was meaningless. Ignatius saw that there was no possible way to align the deception of Docetism with the truth of Christianity.
Within the context of this letter, to "continue in doctrine" means to hold fast to “the faith that was once for all entrusted to the saints” (Jude verse 3). To "go beyond” would be to reject the truth that Jesus was fully human and fully God and to embrace false teachings that deny who Jesus, the Christ, really is. As John Gill puts it, to “abideth not in the doctrine of Christ” is to reject the truth about the person of Christ and to join ranks with Satan. "To go beyond" is to wander away from and reject the truth about Christ Jesus.