2 John 2:19 ifs talking about the “antichrist “ who deny that Jesus is the Son of God. See verses 18 and 22. The word “They” in verse 18 refers to the antichrist who are the ones who have left. Who are the “us” in verse 19? The “us” are the apostles. Read chapter 1:1-3- “That…which WE ( the apostles) have heard, which WE (the apostles) have seen with our eyes, which WE ( the apostles) have looked upon and our hands have handled... WE ( the apostles) have seen and BEAR WITNESS and DECLARE TO YOU that you may have fellowship with US ( the apostles).” Verse 5- “This is the message that we have heard from Him and DECLARE TO YOU..” so, keeping this verse, 19 in the Context in which it was written, the apostles ( the us) are letting the brethren know that these false teachers, the antichrist, who are denying that Jesus is the Messiah, the Son of God, are not apostles, nor in fellowship with the apostles. This
Is not talking about all Christians everywhere, but about a certain group of false teachers who might even have claimed to be apostles.
You have taken this verse completely out if its context and perverted it to teach something the Holy Spirit never intended as anyone who reads the context can plainly see.