How does 1 John 2:19 fit with the teaching that Christians can lose their salvation A common belief (not my own, but fairly common) is that believers may lose or abandon their salvation.  A number of Bible verses seem to contradict this teaching, but in particular, 1 John 2:19 stands out.

1 John 2:19 (NASB):


  They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us.


It seems like John is talking about a group (who he refers to as antichrists) who left their fellowship.  He seems to imply that if they had been true believers in the first place, they would not have left - which seems to directly contradict the doctrine of conditional security.

How do proponents of conditional security interpret this verse, so that it does not contradict conditional security?

I'm looking for either:

(1) A logical, natural alternate interpretation of the passage, or

(2) An answer that documents and references the most prevalent "conditional security friendly" interpretations of this passage, even if those interpretations are not necessarily logical or natural
I think we're reading our cultural bias back into this text - namely by saying that it is talking about salvation in terms of "in" or "out" and our soteriological context. That is a very Western concept, and was largely foreign to the apostles. For them, salvation was found only in the Church (referring here to the gathered and 'sent' people of God, not to a building nor an institution), and to leave the Church was to leave Christ. Granted, this has soteriological implications, but only as a consequence of leaving the Church and remaining outside of Her.

As you stated, the context of this passage is indeed that the αντιχριστοι (antichrists) were teaching false doctrine, and they "went out" from the κοινωνια (fellowship, participatory community, mentioned in 1:3). The verb μεμενηκεισαν implies that they did not remain, abide, reside, continue, stay, endure, and/or were no longer present. We don't have a great English term to effectively translate this concept, so you often see it in all of the above forms (and some others as well). μεμενηκεισαν is in pluperfect active indicative (3rd person plural) here, simply implying that their present absence is a "state or condition following a completed action in the past," i.e. they (voluntarily) went out and so they no longer remain.

The apostle's view was that we come to the Church and we remain in the Church because the Church is the Body of Christ, and to be one with Him is to be united to the Church, which is His Body (1 Corinthians 12:27; Romans 12:4). Thus there is no salvation outside the Church because there is no salvation apart from union with Christ's Body (which is the Church). This is very sacramental (mysterious) and involves how we are "partakers of the divine nature" (2 Peter 1:4). Trying to understand this passage within the worldview of Western Christian philosophy compartmentalizes salvation as a divine singular action that can occur outside of the Church if someone confesses Jesus as Lord and believes several historical facts about Jesus, or simply prays a special prayer. James reminds us that even the demons believe correct doctrine about God (James 2:19) and that "faith apart from works is dead" (2:26).

So to clearly answer the question, this passage has little to do with the doctrines of conditional or unconditional security. The meaning is simply that these false teachers left the Church (the apostolic fellowship), and by doing so proved that they also left Christ; for to be one with Him is to be united to the Church, which is His Body. 

As a further note, the theological implications of this passage have more to say about the concept of apostolic succession than they do about Western soteriology. The real question it should cause us to ask is whether or not we are part of the Church that has remained faithful to the teachings of the apostles.