Does that verse mean that all people who leave the faith were not believers?
John is writing to them because there were a lot of false teachers by this point trying to pull them away from Christs doctrine. It’s about false teachers purporting to be of the church and of God but weren’t they were teaching other things
It’s much clearer reading the entire chapter a few notes
“Little children, it is the last time: and as ye have heard that antichrist shall come, even now are
there many antichrists; whereby we know that it is the last time. They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.”
1 John 2:18-19 KJV
“These things have I written unto you concerning
them that seduce you.”
1 John 2:26
The apostles were constantly guarding the doctrine because there were so many false teachers
johns writings mention it again here
“For
many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is
a deceiver and an antichrist. Whosoever transgresseth, and abideth not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son.
If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed: for he that biddeth him God speed is partaker of his evil deeds.”
2 John 1:7, 9-11 KJV
Peter also writes about it
“But there were false prophets also among the people, even as
there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift
destruction. And
many shall follow their pernicious ways; by reason of whom the way of truth shall be evil spoken of.”
2 Peter 2:1-2 KJV
also paul
“For the time will come when they will
not endure sound doctrine; but after their own lusts shall they heap to themselves
teachers, having itching ears; and they shall turn away their ears from the truth, and shall be turned unto fables.”
2 Timothy 4:3-4 KJV
John is addressing bad doctrine that came to them and those who carry it