I know where it breaks down, too.
It breaks down when someone never had it in the first place, no matter what their religious upbringing.
Just cause you've been in a non-Catholic church that you call *Bible thumpers* does not mean you were ever reborn spiritually.
And if you never had the Spirit of God dwelling in you, then you had no guidance of the Holy Spirit.
1 Corinthians 2:14-16 The natural person does not accept the things of the Spirit of God, for they are folly to him, and he is not able to understand them because they are spiritually discerned. The spiritual person judges all things, but is himself to be judged by no one. For who has understood the mind of the Lord so as to instruct him? Be we have the mind of Christ.
According to Revisionist doctrine that's not possible. And the verse you cite is referring to pagans, not Christians.
You can't have it both ways. If someone is saved according to your doctrine, you can't go back and say they're not when they become convinced your theology is wrong.