6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.
since they crucify again
The verbs “crucify” and “put to an open shame” are temporal. That is, those who fell away cannot be renewed while try to crucify Christ again or put Him to open shame. If these words are causal, then the idea is to show why it is impossible to renew believers.
It is not possible for sacrifices of the Old Testament to renew a person when Christ has already done everything a person needs for forgiveness; He paid completely for all sin of all time on the cross. The sacrificial system of Judaism was abrogated because Christ died fully for all sin. It is impossible to have a repetition of Christ’s crucifixion.
The reason the readers of Hebrews failed to mature was they conflated the ongoing Levitical sacrifices with final and finished sacrifice of Christ. As long as they believed this false notion, they could not accept the final provision of Christ. The attempt to nail Christ to the cross again violated His finished work. Their failure was to co-mingle the crucifixion of Christ with other sacrifices.
Christians cannot be brought to repentance without belief in the finality of the work of Christ.
People cannot be brought to repentance while they treat the cross of Christ in a non-exclusive way. To do otherwise is to disgrace His work of salvation.