Not true..."fallen away" means exactly that...Paul here is referring to some who had fallen away, or apostatized from the Christian faith....they had knowingly made the choice to become apostates, even though they had been “once enlightened"...
Hebrews was written to the Hebrews according to the Author; but why was it written. ???
well the church was being persecuted and the Jewish believers were abandoning the faith, going back to their old religion of Judaism. Paul warned about this and said this was a fatal error for them to do this. we christians can't bring them back cause they gone; however with God nothing is Impossible.
nothing.
a theologian has interpreted this to mean only God can bring them back, so i go with that. i pray for people i have known for decades that have fallen away and i never see again. God can bring them back; just start there and see if that works for you.
a book i study on these issues delves into this, fairly long as the Revision is 500 pages....

...this one is a quicker read, the original by Shank who was 'once saved always saved' but he knows Salvation should lead to the narrow way prescribed by Jesus, Galatians, James et al. in his research as a bible professor he faced the truth and was kicked out of his job as a teacher and a pastor.
so he paid a price that most will not pay. i am the same way, the Truth is more important than just 'go along, get along' mentality. if you have back bone people, this is your book. i have others but this one will clean the clocks of those who are honest and wanna know.

...this revision came out this year and is longer, i use both editions and recommend them both starting with the original.
Salvation is very basic, miss that and you miss everything. get Salvation that leads to holy living and you Found It all they way to Gold Street.
be careful, most people have a scripture twister built into their theology that says we can live however we want to. the devil has been preaching that since forever but it fails at judgement day.
Salvation road is a daily walk with Jesus, initially you come to the Cross and repent, born again. ok, but live for Jesus then, don't go back to the World of Sin et al.
how simple can it be?
read Galatians 5 for a 101 start on this...
1 Corinthians 6:9-10 states: "Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners,
shall inherit the kingdom of God."
Similarly, Galatians 5:19-21 declares: "Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past,
that they which do such things shall not inherit the kingdom of God."
of course some will put this in the Scripture Twister for amazing results, but i don't as we should stand by the Truth.
blessings.
.