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.