For it is impossible, the ones once having been enlightened, both tasting the gift from heaven and partakers of the Holy Spirit, and have tasted of the proper utterance from God, indeed the inherent ability of the coming age, and having fallen away, to be again renewed unto a change of mind, again crucifying for themselves the Son of God, and putting Him to open shame (Hebrews 6:4–6).
The message of salvation—that Christ died on behalf of our sins and was raised on the third day according to the Scriptures—has an inherent ability to save (Romans 1:16). Thus, when a person believes this message, they enter a permanent state of being saved (Ephesians 2:8).
The Christians in Jerusalem had returned to being zealous for the very rules and regulations from which they had been freed by the righteousness that is in Christ (Acts 21:20). Because of this, they were not maturing. Paul encourages them to move on from the basics—not to change their minds again and be re-saved—for a person who has been saved is already in a state of having changed their mind (repented), and therefore cannot be renewed again to this state. That would require that Christ die on their behalf again. Rather, just as the earth is well–spoken of when it produces useful herbs, but the thorns and briers are burned, so it is with the works of a Christian: return to those works which are good.