This passage, in relation to conditional security is apparent when we realize the apostle is writing to believers, as is obvious in the opening verses, where he rejoices that some (vs 4) are walking according to truth, (implying some of the children are not walking in truth). He goes on to speak of deceivers going out into the world, a separate group from the church it seems. (vs 7).
His warning to the believers are to watch, to hang on to the truth, do not lose what you have gained, but to win a full reward.
Then John writes vs 9, where he speaks of one who goes ahead of the true teaching, not abiding in the teaching that has both the Father and the Son.
2 John 1:9
9 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.
The deceivers were against Christ, speaking of Him as only a man, or only as God, not the God-man. (I understand two forms of Gnosticism were rising in the church at the time, either restricting the humanity of the Messiah, or the Deity of the Messiah.
That bit of information was a freeby, since the issue in this post is the responsibility of the believer to maintain the true teaching of the Word, in relation to the Son of God.
It seems John was giving the church a choice. You can abandon the teaching of Christ, and not retain God, or you can abide in the truth and have both the Father and the Son.
I can’t see how John would be addressing two different parties (lost and saved) in the same phrase, unless I was committed to a teaching that didn’t allow this consideration.