To answer this question, you have to go back to the New Testament and the idea of being in the church but not of the church. We see this in 1 John 2:19 when John is talking about false teachers and says, “They went out from us, but they were not of us.” That is still true today.
There are people in the visible church—that is, they attend church and may even be members of the church—but they are not members of the invisible church. They may even make a profession of faith, but we don’t know the heart. We can’t see into the heart. In some denominations, in order to be a member, you have to have a credible profession of faith. They are looking for evidences, but even then they don’t know if someone is genuinely saved.
To sum it up, false converts are not converts who then become not converts. They were never converts. This is not something that’s new for us; it’s there in the epistles of John and in the New Testament church.
Ligonier Ministries