As I understand the entire context and not just the two verses, John couples the anti-christ(ers) with The Anti-Christ, who by definition can never be or never become a believer. Anti-christ(ers) have the same nature as the Anti-Christ. In my Bible there is also a cross reference to Matt 24:24 where these anti-christ(ers) are called false Christs and false prophets who try to mimic the works of Christ.
We don't have to proffer a guess as to who he's referring to in 1 John 2:19.
John himself gives us one definition of these anti-christ(ers) in his next book 2 John 2:7.
"For many deceivers have gone out into the world who do not confess Jesus Christ as coming into the world. This is a deceiver and an anti-christ" (2 John 2:7).
In Revelations, John also uses the word "deceiver" to describe Satan whom Christ calls the "father of liars."
As Euclid said: 'Things which are equal to the same thing are equal to each other.
Anti-Christ = Satan.
Anti-Christ = Anti-Christ(ers)
Anti-Christ(ers) = Satan who was never a believer.
And so, I've come to the conclusion that these anti-christ(ers) were always wolves in sheep's clothing, but others may disagree.
Actually, I think you're saying what I'm saying, but saying it differently.
Phee wrote:If people (talking generalities here) who believed they were saved go out from the body and deny the Son, they couldn't have had the Spirit or they would have known the truth and not denied the Son
I have no problem with differences of opinion; even when I'm right.
( I'm just kidding, but perhaps I shouldn't have written that because I've noticed it's possible to leave out key words when quoting that changes the original statement).
Seriously, since you asked, I hope I've at least given a plausible explanation of what I
believe 1John 2:19 is all about.