Well the fate of backsliders can be a couple of different things, really, depending on what the state of their heart actually is. As Jeremiah says:
"The heart is deceitful above all things, and desperately sick; who can understand it?" (Jeremiah 17:9)
So to what you're saying here, I would say those people may have
thought they believed, but were never
really believers in the first place. True belief in Christ is a matter of the heart and whether one is truly reborn of the Spirit, and thus of God, or not. As John says regarding people who would fall into this category:
"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 it might become plain that they all are not of us." (1 John 2:19)
Or they may just be straying for a time, in which case God, by His Spirit, will bring them back. If God began a good work in someone, He will bring it to completion at the day of Christ (Philippians 1:6).
Grace and peace to you, Cooper.