How do you answer someone who tries to use 1 John 2:19 to say that people who leave God were never really saved in the first place?
If you have had many conversations with people on doctrinal subjects, you will learn eventually that if we so desire we can find "proof texts" for practically any doctrine we like–whether or not is is truly biblical. Such searches for "proof texts" to justify a preconceived idea of what the Bible teaches is not honest, sincere Bible study. I believe anyone using 1 John 2:19 as "proof" that it is impossible to fall away is guilty of just such a misuse of scripture. This passage is neither discussing how one becomes saved, nor is it discussing whether or not one can lose one’s salvation. In the context, John is talking about heretics–people who do not, and presumably never have accepted the biblical picture of Jesus. John calls those who deny that Jesus is the Christ, and those who deny the Son, yet claim to be Christians the antichrist.
If you are having a conversation with a friend over the subject of whether or not one can become a Christian yet later lose their salvation, I believe that 1 John 2:19 is not relevant and is probably not even helpful at all toward deciding this question. In essence, those who abuse this scripture to prove that those who leave God were never saved in the first place are using this passage to create a smoke screen for thier false doctrine.
Of course, in this reply I have definitely NOT established the doctrine of falling away. That is another subject which is answered at the web site in more than one place. However, I believe it is sufficient to answer your question to establish that this passage has nothing to do, one way or another, with whether or not one can be saved and later lose that salvation.
Context, Context, Context!!!