To me, the verse turns on this conditional:
if they had been of us, (a)
they would have remained with us (b)
(b) is tricky because of the word translated "remained" (μένω) which is particularly used in the writings attributed to John:
Mat(3) Mar(2) Luk(6)
Jhn(33) Act(12) Rom(1)
1Cr(8) 2Cr(3) Phl(1)
1Ti(1) 2Ti(3) Hbr(6)
1Pe(2) 1Jo(18) 2Jo(2)
It seems quite likely that throughout John's letters, μένω is at least suggesting a connection to:
Abide in me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in me. I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing. If anyone does not abide in me he is thrown away like a branch and withers; and the branches are gathered, thrown into the fire, and burned.—John 15:4-6 (ESV)
Abide is the same Greek word that is translated remained above. So if John hopes we will think of Jesus as the vine, than the "remained" in
(b) includes both staying part of the community John is writing to and drawing sustenance from Jesus in the spiritual sense. In chapter 2 alone we see several images of what we should "remain" or "abide" with or in:
- Jesus Christ (1st John 2:6,27,28)
- The Light (1st John 2:10)
- Y'all (1st John 2:14,24,27)
- Life (according to the NASB translation of 1st John 2:17)
- The Son and the Father (1st John 2:24)
In each case, there's a reciprocal relationship: each side is "sticking with" the other. There's also a strong contrast with various images of separation in which the relationship is severed or simply does not exist. Of particular interest is the contrast in 1st John 2:15-17 between loving the world (therefore passing away) and doing the will of God (therefore remaining/living).
(a), therefore, is a binary choice: a person is either associated with the world or with God. But in 1st John 2:18-19, we are told that "antichrists" have arisen "from us". They are the subject of the conditional and John is drawing a line between being "from us" and being "of us". In fact, they left "so that it would be shown that they all are not of us." So what was their state beforehand? We can get a hint from the "cover letter" of 1st John:
For many deceivers have gone out into the world, those who do not confess the coming of Jesus Christ in the flesh. Such a one is the deceiver and the antichrist. Watch yourselves, so that you may not lose what we have worked for, but may win a full reward. Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son.—2nd John 1:7-9 (ESV)
The warning signifies that there is some sort of danger. To go back to the vine analogy in John 15, if you don't stick with the teaching of Christ, you will lose the benefit of being associated with him. The sign that a person belongs to one or the other camp is whether they abide. Until a person leaves, the test is whether they "confess the coming of Jesus Christ in the flesh", which seems to be the particular controversy of the community John is writing to.
It all depends on perspective
I have a tomato vine with many branches that are just starting to flower and bear fruit. However, there is one branch that is beginning to separate from the rest of the vine. On the outside, it looks very healthy and strong, but when I trace it back to the source I see that it's in trouble. A strong wind or tug by a bird or animal will pull the branch away from its root system. In a platonic sense, the branch is already dead and will never produce fruit. But until it actually drops off, the branch looks exactly like its neighbors.
John was writing into a similar situation: on the surface the "antichrists" seemed just like all the believers in the community. But they strayed too far from Christ's teachings and to someone with the right perspective, it was obvious they would fall away. When they finally did fall away, John explained that it was because they never had the sort of connection to Jesus that they needed. We know that because if they had the proper connection, they would have remained.
John's emphasis isn't to speculate on whether a person can loose their salvation but to make clear the method of salvation in which we can have confidence:
And now, little children, abide in him, so that when he appears we may have confidence and not shrink from him in shame at his coming. If you know that he is righteous, you may be sure that everyone who practices righteousness has been born of him.—1st John 2:28-29 (ESV)
In other words, we aren't secure in our salvation because of the nature of salvation, but because of the nature of Jesus.