2 John 1:9 — What does it mean to "continue in doctrine?" (Protestantism) 2 John 1:9 (KJV2000) Whoever goes beyond, and abides not in the doctrine of Christ, has not God. He that abides in the doctrine of Christ, he has both the Father and the Son.


Cf. 1 Tim. 4:16.

Question

According to Protestantism, what do "continue in doctrine" and "go beyond" mean, respectively?
First, it might be necessary to establish what doctrine is to be continued in. That very epistle states clearly what doctrine is NOT meant – the Gnostic teaching that Jesus Christ had not come in the flesh. Verse 7 expressly warns all Christians that such a doctrine comes from a deceiver and an antichrist. 

There are only 13 verses in this little epistle, and it appears to be simply warning Christians of the first century A.D. not to depart from, or to leave behind, the doctrine that Christ had come in the flesh, by accepting the opposite doctrine. Verse 10 states, “If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him ‘God speed’.”

The ‘this doctrine’ is the one Christians are to abide in, that Jesus Christ has come in the flesh. Sticking to that truth will protect them from wandering off into another doctrine, a false teaching about Christ. The seriousness of this false doctrine is so clearly stated that any Christian accepting it would be viewed as having so gone beyond it as to have revolted against the truth of God and Christ, and would no longer be received into Christian homes, or given a blessing by them.

The Protestant study notes for the NIV translation of 2nd John make the point that, 


  “During the first two centuries the gospel was taken from place to
  place by travelling evangelists and teachers. Believers customarily
  took these missionaries into their homes and gave them provisions for
  their journey when they left. Since Gnostic teachers also relied on
  this practice (see 3 John 5), 2 John was written to urge discernment
  in supporting travelling teachers; otherwise, someone might
  unintentionally contribute to the propagation of heresy rather than
  truth.”


I am a Protestant myself and entirely agree with that explanation.  It is also worth pointing out (from a Protestant point of view) that Jude’s little epistle, in verse 3, tells Christians of the first century to “earnestly contend for the faith which was once delivered to the saints”. Past tense, note. And Hebrews 1:1-3 shows that, by then, God’s final word had been spoken, through Jesus Christ who had now returned to Heaven. These verses have long served as a warning to Protestants not to heed new doctrine (teachings) beyond what had already been received by the Christian Church by the end of the first century A.D. 

Another word for doctrine of the apostles is ‘tradition’, by the way. It was only from the middle of the second century A.D. that the word 'tradition' came to mean something different from its use in the Christian Greek Scriptures (the New Testament). Whereas those scriptures recorded the tradition (teaching) of the apostles, later it began to mean the teachings of the early Church Fathers. At that period tradition gradually changed to mean the interpreting and unravelling of Scripture to stand over against Scripture, as expounded by the writings of the early Church Fathers. Obviously, the apostle John did not have that in mind when he wrote his 2nd epistle.

Summary: To ‘continue in doctrine’ means to believe such teachings, to keep them intact, without adding to or taking from them – preserving them as is.
To ‘go beyond’ such doctrine is to either inflate it with additional points, or to leave behind the Christian teaching due to having taken up new, un-Christian teaching.