There are two errors to make on this question, and a razor’s edge to walk to answer it correctly. Just as the Bible teaches both divine sovereignty and human responsibility, so it also teaches both that apostasy is real, and that no believer could ever lose his salvation. We must deny neither that apostasy can and does happen, nor that once we find forgiveness in Christ that we can never find ourselves unforgiven.
What then is apostasy? It is not an ontological believer becoming an ontological unbeliever, but a phenomenological believer becoming a phenomenological unbeliever. Clear enough? Ontological and phenomenological are fifty-cent words that have fifty-cent meanings. Ontological means being while phenomenological means as perceived. Clear enough? Let’s try again. Ontological means as the thing really is, while phenomenological means as the thing appears to our senses. When we say the earth rotates on its axis, we are speaking of how the reality is. We are speaking ontologically. When we say the sun rises in the east we are saying as it appears to our eyes. We are speaking phenomenologically.
Apostasy then is when a person who appeared to be a believer to the naked eye, who professed Christ, who was believed to be a believer no longer appears to be a believer to the naked eye. This is consistent with the biblical doctrine of the perseverance of the saints, including this critical text from John:
“They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us” (I John 2:19).
Notice that John says their status of not belonging didn’t change, but became “manifest” which means seen or known. Apostasy is when our denial of the faith makes evident that we do not have faith, and, that we never actually had it to begin with.
Why then is it such a dreadful thing? It’s not as if apostates have lost something they once had. For this simple reason- it is a dreadful thing for anyone to be outside of the kingdom. How much more so for someone whom we thought to have been in the kingdom? In addition, there are other texts that, at least to some, suggest that an apostate cannot, or perhaps better said, will not be brought to saving faith. See for instance Hebrews 6 and 10. There are differing views on these texts.
What we ought to be confident in are these sure and certain promises of God- “He who has begun a good work in you will complete it until the day of Jesus Christ” (Philippians 1:6) and “My sheep hear My voice, and I know them, and they follow Me. And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand” (John 10: 27-28). All those in Christ are secure in Him. No power, including our own wills, can snatch us from His pierced hands. Let us then be about the work of making our calling and election sure, knowing it is He that works in us both to will and to do His good pleasure (Philippians 2:13).