“In context, the “deliberate sin” he’s talking about is apostasy (return to Judaism) motivated by cowardice. But the point stands that it’s possible to fall away.”
It is not possible for the Elect to fall away, as is quite clear in scripture, though the reprobate can certainly fall away. As Augustine explains:
“But of such as these [the Elect] none perishes, because of all that the Father has given Him, He will lose none. John 6:39 Whoever, therefore, is of these does not perish at all; nor was any who perishes ever of these. For which reason it is said, They went out from among us, but they were not of us; for if they had been of us, they would certainly have continued with us. John 2:19”. (Augustine, Treatise on the Predestination of the Saints)
Hebrews 6 most likely deals with men like Judas, who was filled with the Spirit, and even performed miracles along with the rest of the Apostles (Matt 10:1-8), and yet was always accounted a devil and a thief (John 6:70, 12:6). There are men during the Apostolic age who performed miracles by the power of the Holy Ghost, and yet had no saving grace, and therefore were ultimately unknown to the true church of God.
“Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.”
(Mat 7:22-23)
Compare to Christ’s words to the SHeep of God, who are always known by God:
“My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.”
(Joh 10:27-28)
And again, all those who apostasize from the faith, were never of us from the beginning:
1Jn_2:19 They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.
Depends on who "the Elect" are. God knows from all time who is ultimately going to heaven and who isn't. It is impossible for the blessed to fall away and be damned, because then they wouldn't be the blessed anymore -- but that observation is tautological.
Those are the "elect" Augustine is talking about.
But Augustine also explains that they are seen (by us) only to have been preserved in hindsight, *after* death:
I assert, therefore, that the perseverance by which we persevere in Christ even to the end is the gift of God; and I call that the end by which is finished that life wherein alone there is peril of falling. Therefore it is uncertain whether any one has received this gift so long as he is still alive. For if he fall before he dies, he is, of course, said not to have persevered; and most truly is it said. How, then, should he be said to have received or to have had perseverance who has not persevered? For if anyone have continence, and fall away from that virtue and become incontinent,—or, in like manner, if he have righteousness, if patience, if even faith, and fall away, he is rightly said to have had these virtues and to have them no longer; for he was continent, or he was righteous, or he was patient, or he was believing, as long as he was so; but when he ceased to be so, he no longer is what he was. On the gift of perseverance, ch 1Augustine seems here to be clearly saying that some people "are rightly said" to have faith and various virtues, but then can fall away. He doesn't seem to endorse the "they were never really saved to begin with" explanation.