The following is from a public post by Robert A. Gagnon:
“Watch yourselves, in order that you may not lose what we [or: you] worked for, but receive a full reward.” (2 John 8 )
The importance of “remaining in Jesus” as a requirement for receiving eternal life is important to the vine-and-branches analogy in John 15. Jesus emphasizes that those who believe in him must "remain (abide, continue, stay) in me" (v. 4). The very concept of remaining presumes continuance in a genuine condition, not continuance in appearance only (what is the point of remaining in fake discipleship?). "If anyone does not remain in me, he is thrown out like a branch and dries up, and they gather them and throw them into the fire and they are burned" (v. 6) -- a clear illustration of the falling away of once-genuine believers who fail to exhibit a transformed life.* Only "if you keep the commandments" do you "remain in my love" (v. 10).
The importance of "remaining" in Christ is also emphasized by the Elder John in the Johannine Epistles that are so closely related to the Gospel of John.
In 2 John 8 the Elder John warns his readers: “Watch yourselves, in order that you may not lose what we [or: you (with other mss.)] worked for, but receive a full reward.” The command to “watch” (blepete) appears four times in Jesus’ Eschatological Discourse in Mark 13 (vv. 5, 9, 23, 33) to warn against dangers that could lead to being excluded from God’s future kingdom.
The context also indicates loss of salvation, the “full reward” of what they are experiencing only partially now (cp. 1 John 3:2: though “we are now children of God,” only in the future when Jesus “is revealed will we be like him”; also 2:28). The preceding verse refers to “deceivers … who do not confess Jesus Christ as coming in the flesh,” also designated “antichrist” (Docetism). The following verses warn against welcoming anyone “who does not remain in the teaching of Christ” and so “does not have God,” for to do so is to “partner with his evil works” (vv. 9-11).
There may be an allusion here to John 6:27-29, where Jesus exhorts the crowd following him to “work for … the food that remains to [or: for] eternal life” and then defines “the work of God” as “believing in the one whom that one (God) sent.” Needless to say, the concept of “losing” implies prior possession, and the reference to “work” (even in its identification with faith in Christ) indicates requisite human action cooperating with God’s gracious offer of salvation.
In 1 John 2:24-25 the Elder makes “remaining in Jesus” and receiving the promise of “eternal life” contingent upon converts “remaining” in the gospel message and leading a transformed life.
"Let (i.e., see to it that) what you heard from the beginning remains in you. If what you heard from the beginning remains in you, you too will remain in the Son and in the Father. And this is the promise that he himself promised to us, eternal life." (2:24-25)
Again, the very concept of remaining (staying, abiding, continuing; Gk. meno) implies that a process had already begun. What is being required is continuance.
There is no question here of his audience being fake believers. Were they fake believers, they would not be exhorted to continue in this condition. Throughout 1 John we find expressions of confidence that their start in the faith was genuine, including in the immediate context:
"And you have an anointing from the Holy One, and you all know (i.e., have knowledge). I have not written to you that you do not know the truth but that you know it.... And the anointing that you received from Him remains in you, and you do not have need for anyone to teach you, but as his anointing teaches you about all things, and is true and not a lie, and just as it taught you, remain in him [or: it]. And now, little children, remain in him, so that if (i.e., when) he is made manifest (is revealed, appears), we may have boldness and not be ashamed away from (i.e., before) him at his coming" (2:20-21, 27-28)
Clearly, then, the believers have the anointing of the Spirit, but are still cautioned to “remain in” Jesus; for otherwise they will be separated “away from” him and “ashamed before him at his coming,” meaning loss of salvation (cp. Mark 8:38).
When the Elder John exhorts his readers to have the gospel remain in them and makes their reception of eternal life contingent upon the same, John is indicating the possibility that they too, while having begun well, could finish badly if they do not hold firmly to the gospel message (2:2; 4:10; see also 1:7). Ch. 2 begins and ends with another way in which the believers must "remain," a theme that thereafter persists: They must continue to keep Jesus' commandments (2:3-17, 29).
"The one who says that he remains in him ought, as that one walked, also himself to walk in this way" (2:6)
"The one who does the will of God remains forever" (2:17b)
"Everyone who remains in him does not keep on sinning" (3:6a)
"The one who keeps his commandments remains in him and he (God) in him" (3:24a)
"If we love one another, God remains in us" (4:12b)
"The one who remains in love remains in God, and God remains in him" (4:16b)
The author can even say that "everyone who hates his brother ... does not have eternal life remaining in him" (3:15). This destroys the "logical" deductive reasoning that if eternal life is to remain eternal, it cannot be lost. In fact, it can be lost because eternal life is located in the Son, and if one ceases to remain in the Son, so too does eternal life depart from that one.
All of this has ramifications for how best to read a favorite proof text for the OSAS and POTS positions: 1 John 2:18b-19:
"Many antichrists have appeared, from which we know that it is the last hour. They went out from us but they were not from us (i.e., did not belong to us, were not part of us). For if they had been from us, they would have remained with us (till now) -- but (this happened) in order they might be exposed [or: made manifest, revealed, shown, known] (for what they are, namely) that they are all not (i.e., that none of them are) from us."
These "antichrists" who left John's community are further described as persons "denying that the Christ (Messiah) is Jesus" (2:22; cp. 2 John 7). Possibly, as with later gnostics, they viewed "the Christ" as merely inhabiting Jesus' body without any intrinsic connection, where Jesus is just the body-shell of the spirit-being Christ.
On the surface it seems as if John is saying that, had these antichrists been genuine believers, they would never have departed from the true faith: once saved, always saved. As it turns out, this is an overread of 2:19.
When John states that by "going out from us" the antichrists show that they "were not from us," and that "if they had been from us, they would have remained with us," he is not asserting that genuine believers can never cease to remain in Christ. That interpretation would contradict the rest of the letter.
Rather, John means that believers who genuinely believe in Christ but subsequently depart from this belief and from a transformed life were never really genuine believers in the sense of being “abiding believers.” By leaving John's community and changing over to a heretical gospel and a life of sin, these antichrists show not necessarily that they never believed an orthodox gospel and lived a transformed life but rather that they couldn't "remain" (continue, stay, abide) in that condition.
Only those who so "remain" in the true faith till Christ returns are truly "from us." If they had truly been "from us" (belonged to us, were part of us), they not only would have begun well; they would have finished well. Whether they never really believed and lived a transformed life, or they once did but didn’t “remain” in such, they still fall under the heading: “They were not from us.”
Further confirmation for the view that the Elder John believed that once-genuine believers could fall away comes from 5:16-17, which warns of unpardonable mortal sin:
“If anyone sees his (or her) brother (or sister) sinning a sin (that does) not (lead straight) to death, he (or she) will request and he (God) will give to him (or her) life, to those who are sinning not to death. There is a sin (that leads straight) to death. I am not saying that you should ask about that. All unrighteousness is sin but there is a sin (that does) not (lead) to death.”
Based on themes elsewhere in 1 John, mortal sin probably includes:
(1) An unrepentant denial that Jesus is the Christ or Son of God and has come in the flesh (1 John 2:22; 4:2-3, 15; 5:1, 5, 12; 2 John 7);
(2) Egregious instances of not keeping the commandments (1:6; 2:4; 3:4, 8, 10; 5:2-3), including not loving fellow believers (2:9, 11; 3:10, 14-17; 4:20-21) and being controlled by worldly desires for sex and money (2:15-17).
The passage with which we started the discussion of the Johannine Epistles, 2 John 8, appears to be a warning about committing such a mortal sin, and so losing what was worked for and failing to receive “the full reward” of eternal life.
*Obviously, the warning about being removed from Christ the Vine in John 15 would lose its urgency and potency if the Eleven believed that they could never really be removed. The warning is real. The Eleven can be confident about their future fate, but not presumptuous. Their ongoing relationship with Jesus is still conditional. They have to continue to acquiesce to Christ’s work in them, bearing the fruit that comes from keeping his commandments. If they don’t, they face removal from Christ. There is no “then they were never genuine believers” operative here. Whatever is meant by Jesus’ words in 6:37-45 and 10:28-29, these texts do not contradict the clear warning in John 15:1-17.