Albert Barnes Commentary Hebrews 10:30

"For we know him that said, Vengeance belongeth unto me, I will recompense. And again, The Lord shall judge his people." — Hebrews 10:30 (ASV)

For we know him who has said. We know who has said this—God. They knew this because it was recorded in their own sacred books.

Vengeance belongeth unto me, etc. This is found in Deuteronomy 32:35. See it explained in the commentary on Romans 12:19.

It is quoted there to show that we should not avenge ourselves; it is quoted here to show that God will certainly inflict punishment on those who deserve it. If any should apostatize in the manner the apostle refers to here, they would, he says, be guilty of great and unparalleled wickedness and would have the certainty that they must meet the wrath of God.

And again, The Lord shall judge his people. This is quoted from Deuteronomy 32:35.

That is, he will judge them when they deserve it and punish them if they should be punished. The mere fact that they are his people will not save them from punishment if they deserve it, any more than the fact that one is a beloved child will save him from correction when he does wrong.

This truth was abundantly illustrated in the history of the Israelites. The same great principle would be applied should any sincere Christian apostatize from his religion: he would have before him the certainty of the most fearful and severe of all punishments.