Eternal Security, simplistically known as "once saved always saved", contends that salvation is a permanent thing that can't be undone. But what about those who lose faith? What about ex-Christians? Are they still saved? The key word there is "still". Eternal security may be commonly stated as "once saved always saved", but as I said, this is a far too simplistic way of saying it. The truth is, those who leave the faith aren't still saved because they were never saved to begin with.
It's one thing to believe in God. It's even one thing to believe in Jesus. As James tells us, even demons believe, and shudder (James 2:19). Experiencing true salvation, however, is another thing entirely. Salvation requires more than just intellectual assent; it requires a relationship.
In Hebrews 10:39, Paul draws the distinction between those who believe, to the saving of the soul, and those who draw back to perdition. These two groups, scripture says, are completely distinct from each other. A similar statement is found in 1 John 2:19: "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."
The Hebrews verse strongly implies that those who believe are not like those who draw back to perdition. The 1 John verse tells us far more explicitly that those who are "of us" (i.e. those who believe, to the saving of the soul) would have stayed with us if they were truly of us. Scripture, then, is clear: There is no such thing as an ex-Christian. Once saved, it becomes impossible to become unsaved, simply because God will not let you go. So yes, "once saved always saved", but that actually involves getting saved in the first place.