I just completed bible study on Hebrews. However, we only touched on this a little, but here’s what it said.
In 6:4-6 is talking about those who received Christian initiation (ie Baptism, Eucharist, and confirmation) and then reject Christ’s sacrifice.
Later, in 10:26-27, we see a similar statement. “If we sin deliberately after receiving knowledge of the truth, there no longer remains sacrifice for sins but a fearful prospect of judgment and a flaming fire that is going to consume the adversaries.”
Our commentary says that for the Jewish Christians having embraced Christian faith, that if they now reject Christ’s sacrifice for sins, there is no other sacrifice because the sacrifices of the old covenant have been superseded by the all-sufficient sacrifice of Christ (which they now reject). That is, they can’t go back to their old ways (the old covenant sacrifices). For sins to be forgiven, they must accept Jesus’ sacrifice. The author is using this as a warning to the community he was speaking to, so they recognize the seriousness of their situation if they decide to reject Christ.
It doesn’t mean that there is an unforgiveable sin if you accept Christ. Only if you reject Him.