What does Hebrews 10:31 mean?

The prior verse quoted from the Old Testament, where God had promised to seek vengeance on nations who harmed Israel. However, those promises were part of a warning, also from God, about what would happen if Israel sinned. In such cases, the nation would be subject to God's wrath, in the form of attack by those foreign nations.

In prior verses, the writer has indicated that those who know the truth—saved Christian believers—are subject to particularly harsh punishment from God if they choose to disobey. This is not a loss of salvation; rather, it's a consequence for sin similar to what Israel suffered in the wilderness (Hebrews 3:12–19). Earlier warnings included a reference to God's judgment in the form of fire (Hebrews 6:1–8). Earlier in this passage, this idea of God's wrath was again connected to flames (Hebrews 10:27). Since believers know, better than anyone, what the sacrifice of Christ is worth, God will respond to those who "profane" His covenant through willful sin all the more harshly.