Apostasy, or falling away, is a knowing and settled rejection of Christ after deep exposure to the gospel; it reveals that the Word never became rooted life, not that rooted life was lost.
The New Testament’s severe warning passages describe those who, having been enlightened and having tasted the good word of God, deliberately turn away. This is not ordinary failure, doubt, or a season of disobedience that troubles a true believer; it is defection — the revelation of unrootedness, as the seed on rocky ground springs up and withers.
Those joined to the risen Christ do not revert to what they were; but the warnings are real, and God uses them to keep His people from ever drawing back.
See in Scripture: Hebrews 6:4–6; Hebrews 10:26–29; Luke 8:13; 1 John 2:19