Hebrews 10:30

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

Vengeance belongeth unto me ... - This is found in Deuteronomy 32:35; see it explained in the notes on Romans 12:19. It is there quoted to show that we should not avenge ourselves; it is here quoted to show that God will certainly inflict punishment on those who deserve it. If any should apostatize in the manner here referred to by the apostle, they would, says he, 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 32:36. That is, he will judge them when they deserve it, and punish them if they ought to 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; and 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.