I believe that this is speaking of a very distinct scenario where a non-elect person becomes reprobate, which means they are turned over to their sin after first tasting, then mocking the grace of salvation. I do not believe a true believer can lose their salvation. But there is a serious warning here. Just because you are in the garage, it doesn't make you a car. Just because you go to church, it doesn't make you a Christian.
Why does it shock you when people leave Christ? Many left him in the gospels and no longer followed him after he told them hard things that they didn't want to hear. Jesus himself said:
Let us continue in faith and faithfulness and show ourselves to be very far from these warnings for reprobate hypocrites!
In Prayer Today:
Seek the face of God for assurance today of your salvation by grace through faith in Christ!