Heb 6:4-6 merely shows how far a hypocrite can go and yet not actually be saved. If it mentioned someone who was born again, or regenerated, or a child of God, then one could make a case for a fall from grace, but it doesn't.
Just take him back to John 3:16 and 1 John 5:13. Once one has everlasting/eternal life it can't end otherwise it would not be everlasting. With the doctrine that denies eternal security, one can never HAVE everlasting/eternal life. Then the whole game is over.
Quote
1 John 5:13 These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
John 3:16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
