You just said it, by inferring (incorrectly) that freedom from sinning is unscriptural.
David was still a man of the flesh.Believers in both testaments sinned.Some times severely.David had Uriah killed and took his wife.
The option to walk in the Spirit wasn't available till after Jesus was raised from the dead.
Peter denied the Lord 3 times.Peter still walked in the flesh. He was still a Jew by religion.
Not for sin, but for allowing his actions to imply the Law was still in effect.He was rebuked to his face by Paul.
The unrepentant are not converted. He was still a man of the flesh instead of a man of the Spirit.Simon in Acts 8 sinned and Peter told him to repent and pray.
That isn't a believer.Believers who took the Lord's Supper unworthily got sick and even died.
It is an unbeliever.
Not "his" Spirit, but the church body/Spirit.Paul delivered the Corinthian fornicator to Satan for the destruction of the flesh that his spirit might be saved in the day of the Lord Jesus.The man later repented.
Repentance was offered to him, but we don't know that he accepted it.