Commenting on Heb 6 and how it clearly presents regenerated/true Christians as losing their salvation, I. Howard Marshall wrote:
[T]he conclusion is irresistible that real Christians are meant. The most that J. Owen is able to say is ‘that the persons here intended are not true and sincere believers in the strict and proper sense of that name, at least they are not described here as such.’ He states that there is no mention here of faith. This is surely quite irrelevant, for Christians may well be described and defined from other points of view; there is no mention of faith, for example, in the description in 1 Corinthians 1:1-10, yet nobody doubts that Christians are there depicted. Owen holds that no characteristic peculiar to believers is described here, but, if our exegesis is sound, what is stated here is characteristic of believers and only of believers. He draws a contrast with the true believers described later in the chapter. But the contrast is surely not so much between two different groups of people as between two possibilities which may affect the same people; thus verses 7f. describe two possibilities which may arise in the same land.
The lapse of these believers is indicated by the verb παραπιπτω which here must mean ‘to fall away from the faith.’ Although the corresponding noun παρααπτωμα means a single lapse or act of sin, and although the passage was later so understood by the Montanists and Novatians, it is difficult to believe that one single act is described here. A total renunciation of Christianity is meant, and the nature of such an act is perhaps intentionally left vague: ‘The writer never hints at what his friends might relapse into. Anything that ignored Christ was to him hopeless.’
Two further participles now describe more precisely what is involved in the action of the lapsing. To lapse is to crucify Christ to oneself and thus to put Him to ignominy. The verb ανασταυροω probably means ‘to crucify again,’ and implies that the lapsed are metaphorically repeating the act of rejection perpetrated by the Jews at Golgotha. ‘Nothing more shameful could be committed than to reject the Saviour.’
It is generally held that these participles are causally related to the main verb ‘to renew’; men who fall away cannot be restored because what they are doing is to crucify Christ. The Revised Version margin translates ‘the while they crucify . . . ‘which would express the fact that repentance is excluded as long as they continue to crucify Christ; on this view the passage would not absolutely exclude the possibility of restoration to repentance, but would simply state that continuance in the state of apostasy will lead to destruction . . . We should note, however, that the persons throughout of apparently have no desire to repent. The Epistle as a whole is a summons to repentance and the people indicated here have no desire to be forgiven for their sins; they have rejected the only One who can give them the opportunity of repentance.
Finally, the author uses an analogy to indicate the reasonableness of what he has said. If a field receives rain from heaven and brings forth useful crops it receives a blessing; not merely from men but also from God; but if after receiving rain it produces weeds, it is regarded as reprobate, and is liable to be accursed and will be burned.
Despite this strong language, however, the writer is convinced of better things regarding his readers, and his language now turns to encouragement. They already show signs of things which are connected with salvation, and he longs or them to continue to display such signs. Hence, although he has warned his readers of what their sluggishness may lead to, he is on the whole more confident that they will press on to salvation and he urges them do to so lest through failure to progress they should perhaps all back into danger. (I. Howard Marshall, Kept by the Power of God: A Study of Perseverance and Falling Away [Minneapolis, Minn.: Bethany Fellowship, Inc., 1969], 144-45)
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