Those in Heb 6 that fall away, it says of them "seeing they crucify to themselves the Son of God afresh, and put him to an open shame."
The present tense of the verbs 'crucify' and 'put' show they did not simply fall away and nothing more but they fell away and CONTINUED to crucify Christ and put Christ to shame. Christ is the reason one repents but if one CONTINUES to crucify and put to shame the REASON (CHRIST) that one repents then repentance would be impossible. Yet if one quits CONTINUING his crucifying and putting Christ to shame he can then come to repentance. Acts 8, Simon did not continue in his sin against the Holy Spirit but was able to repent.
Also, the book of Hebrews is filled with warnings and admonishments to those converted Jews about falling away, going back to the OT leaving the NT and the only possible way to be saved. They were warned about drifting away, Heb 2; departing from God in unbelief, Heb 3; falling short of that rest, Heb 4 all of which is needless and unnecessary if falling way were impossible. They had not matured as Christians as they should Heb 5 and were full of hearing. They were true, genuine Christians Heb 6:4-5. They were warned about falling into a state where they would CONTINUE to crucify and put to shame Christ then repentance would be impossible.....
"Both of the expressions “crucify” and “put him to open shame” are present tense participles. In Greek, the present tense has more to do with the type of action, rather than time (the latter being secondary). The present tense represents an action that is in progress, and generally, one that is sustained (see Wallace, 518).
The “crucifying” and “putting to open shame,” therefore, represent on-going actions on the part of apostates.
It is also important to note that present participles normally express action that is contemporary with that of the main verb of the sentence (Wallace, 625), which, in this case is “renew.”
In other words, “while they continue to crucify,” “as long as they are crucifying,” etc., the Son of God, they cannot be brought to repentance.
Why is this the case? Because Christ is the motive for repentance! How could one possibly repent of falling away from the Christian faith, if he believes that the crucifixion of Jesus was a just sentence upon a false Messiah?
As F. F. Bruce expressed it: “Those who repudiate the salvation procured by Christ will find none anywhere else” (149).
And so, it is not the case that Jewish Christians who abandon the faith cannot ever be saved. The tragic reality is this. They cannot be saved if they drift into a state of unbelief and remain that way!"