Hebrews 10:30

οἴδαμεν γὰρ τὸν εἰπόντα.… “For we know Him who said, vengeance is mine, I will repay.” The certainty of the punishment spoken of is based upon the righteousness of God. “We know who it is that said”; it is the living God (Hebrews 10:31). The quotation is from Deuteronomy 32:35 not as in the LXX but as given in Romans 12:19 where it is used as an argument for the surrender of private vengeance. In Deut. LXX the words are Ἐν ἡμέρᾳ ἐκδικήσεως ἀνταποδώσω. The second quotation, κρινεῖ κύριος … is from the following verse where the words intimate God's protecting care of His people, using κρινεῖ in the sense common in O.T. Delitzsch thinks that sense may be retained here, but this is less relevant and consistent with the passage. Cf. Sir 27:28 ἡ ἐκδίκησις ὡς λέων. and Sir 28:1. φοβερὸν τὸ ἐμπεσεῖν.… “It is dreadful to fall into the hands of the living God”. Where David (2 Samuel 24:14) prefers to do so [ἐμπεσοῦμαι δὴ εἰς χεῖρας κυρίου] it is because he knows his chastisement will be measured and that no unjust advantage will be taken. The dreadfulness of the impenitent's doom arises from the same certainty that absolute justice will be done. As Judge, God is “the living God,” who sees and has power to execute just judgment, cf. Hebrews 3:12; Hebrews 12:22, cf. Hebrews 12:29.