Very good question, @irepchrist242. I believe there is a fundamental difference between the two cases you have mentioned. One turned his back on the faith itself, while the other “is still a Christian but seems to backslide into sexual sin.”
When someone departs from the faith and blatantly denies Christ and His word, they reveal what Hebrews 3:12 calls an evil heart of unbelief in departing from the living God. Jesus said there would be tares among the wheat, goats among the sheep, and that many would call Him Lord in the judgment who were never truly His.
So people who claim to be former Christians are really just former counterfeits finally showing their true colors. I John 2:19 confirms this: they went out from us, but they were not of us, for if they had been of us, they would, no doubt, have continued with us, but they went out that it might be made manifest that they were not all of us.
I think it’s significant that people who fall away, or apostasize like this will often say things like, “I finally decided to quit living a lie”, “I couldn’t keep pretending”, “I never really believed all that stuff”, and similar statements revealing that their hearts were never genuinely transformed by the new birth of the Spirit from the start.
But someone who doesn’t deny the faith, who falls into sin through weakness of the flesh, is a different story – it’s the story of Peter whose faith may waver but doesn’t apostasize.
It’s the story of Lot. Remember him? The guy who moved his family into Sodom – who married his daughters to Sodomites – even called the Sodomites “brethren”, and offered his daughters to them to be gang raped – the guy who lost his wife, got drunk in a cave and… well, you can read the whole sordid mess in Genesis 19.
And if all you had to go on was what you read in Genesis, you’d probably be convinced that there was no way this guy was a follower of God.
But then you turn down to II Peter 2:7-9, and you read in verse 7 where God calls him a “just” man; verse 8 calls him “righteous” - twice - verse 9 even calls him godly! Lot? Godly?
Oh yes, because God saw that Lot’s heart was never really on board with all the sin around him. He was vexed from day to day by what he saw and heard. Because a genuinely saved child of God can never be okay with sin. His heart will grieve over it even as his weakness to the flesh gives into it.
This is why the Bible nowhere ever describes God’s people as wicked. God sees us as having been made righteous through the miracle of the new birth on the inside. The “old man” we used to be within is crucified with Christ (Romans 6:6) – dead and buried, never to return – and a new man has been born inside us.
But the death and resurrection of that “inner sinner” has not yet happened with our outer man – the flesh. Someday it will, and then we’ll be wholly holy. But in the meantime we struggle with the soul of a saint inside the body of a sinner – the nature of God on the inside and the nature of Adam on the outside.
And so, while our hearts want to be at peace, someone may make an insulting comment, and our flesh churns out adrenaline that makes us want to swing a fist – but then our spirit has to say, “No, flesh! Down, flesh! Bad, bad flesh!”
We want to be pure, but some bikini babe goes bouncing by, and our flesh churns out hormones that disturb our spirit into saying, “Bad flesh! Down, flesh! No, no flesh!”
Christians never sin because they are wicked, they sin because they are weak – as Jesus recognized in His sleeping disciples, The spirit is willing but the flesh is weak.
I hope this may help you put things into perspective.