These verses you post in no way suggest one who is saved already, can ever be unsaved. It is about people who heard the truth within the church and set it aside to go back to the law for justification, it is also about encouragement and great information, a massive difference in belief between those under grace and those who have heard of grace but made it of no effect by adding leaven and ruining the whole gospel.
" 9and now, having known God — and rather being known by God — how turn ye again unto the weak and poor elements to which anew ye desire to be in servitude? 10days ye observe, and months, and times, and years! " - Galatians 4:9-10 YLT
He told these saved people that they make Christ of no value to them in justification by doing that.