Find my first three entries on Hebrews 6 here.
Perhaps I owe more commentary concerning R. Bruce Compton's essay ("Persevering and Falling Away: A Reexamination of Hebrews 6:4-6" Detroit Baptist Seminary Journal : 135-167) than I will offer here. I cite his brief comments to refresh your memory what he says concerning the warning in Hebrews 6.
The expression “fall away” is the fifth in a series of five parallel participles which begin in v. 4. These five participles are joined by simple conjunctions and are preceded by a single article. While an adverbial participle can introduce the protasis of a conditional clause, such does not appear to be the case here. The first four of these five participles are invariably taken with the article as adjectival and, specifically, as substantives in a series of relative clauses (“those who have once been enlightened and have tasted . . .”). Because all five appear to be parallel, there is every indication that the fifth should also be taken with the article as an adjectival substantive, continuing the series of relative clauses (“and have fallen away”). As such, it would not be adverbial and, hence, not conditional (pp. 141-142).
All my responses, posted here, to John Sproule's initial brief essay that Compton cites apply. I will recast those observation in a summary form here.
First, Compton assumes that Sproule's analysis of the passage is correct that the plural article (τοὺς) renders the first four participles adjectival (substantival) and that, therefore, the article should be viewed as connected to the fifth participle also, rendering it adjectival rather than adverbial. If Compton's assumption is correct, then the fifth participle (παραπεσόντας) means "those who have fallen away." As such, the passage would not properly be regarded as a warning, grammatically speaking, for the passage would be retrospective rather than prospective. This is not to suggest that the passage could not serve a warning function. However, its retrospective warning function would be akin to pointing out the disaster that came upon Esau (Hebrews 12:16) or to calling attention to the calamity that came upon Ananias and Sapphira (Acts 5:1-10).
Second, if Compton is correct that the plural article must be taken with each of the five participles, then the adverb "once for all" (ἅπαξ) must also be taken with each of the five participles. The result would be that "those who were once enlighted" are being described further as "those who once tasted the heavenly gift," "those who once were made partakers of the Holy Spirit," "those who once tasted the good word of God and the powers of the coming age," and finally as "those who once fell away." Such a reading is dubious.
Third, against Compton's reading of the passage, who follows Sproule's analysis, it seems more likely that "those who were once enlightened" is the overarching description that finds greater expansion and explanation as follows: "Those who were once enlightened, having tasted the heavenly gift and having been made partakers of the Holy Spirit and having tasted the good Word of God and the powers of the age to come, if they fall away it is impossible to renew them again unto repentance because they recrucify the Son of God for themselves and subject him to open shame."
Fourth, as I suggested in my response to Sproule's essay, it seems more likely that τοὺς ἅπαξ should be taken solely with the first participle, "enlightened" (φωτισθέντας), and not with the four subsequent main participles. The reason for this is that if the author wanted all the main participles to be governed by τοὺς ἅπαξ, readers would at least expect a repetition of ἅπαξ, as in Hebrews 9:26-28 (English/Greek), if not a repetition of τοὺς also, since ἅπαξ stands after the plural accusative article.
Fifth, only the fifth participle, "having fallen away" (παραπεσόντας), is negative. As such, it does not at all fit the other four participles, which are all positive, assuming for the sake of Compton's argument that they all characterize or describe the same people: (1) those once enlightened; (2) those who once tasted the heavenly gift; (3) those who once were made partakers of the Holy Spirit; (4) those who once tasted the good word of God and the powers of the coming age; and (5) those who once fell away. This is a strange mixing of negative with positive characterizing of people. No wonder those who adopt this view color the first four participles with a negative tone rather than positively by spilling the negative of the fifth participle onto the first four participles.
It seems to me that Compton's interpretation, which he adopts uncritically from Sproule, fails to account adequately for the structures of the text which I will address in future entries.