Heb 6:4-6 is pretty simple to understand in light of the entire NT. It's obvious that God forgives, even when we knowingly sin.
Jesus commands us to forgive someone 70x7 times in one day, even if it's the same offense, and God would never ask us to do something He wouldn't do.
In light of that, fall away means that we turn away from the truth, and try to find reconciliation to God in that "fallen away" state. There is no repentance until we first rectify the "falling away" - until we return to Calvary.
This is why it says concerning these people that the reason it is impossible to renew them to repentance is that they are crucifying him afresh with their actions, and putting Him to shame on an ongoing basis.
Until they make the decision to stop crucifying Him there can be no repentance.
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