The work of Christ makes us clean in a way the law never could. the promise is scattered throughout the Old Testament, especially in the prophets.
“Come now, let us reason together, says the Lord:
though your sins are like scarlet,
they shall be as white as snow;
though they are red like crimson,
they shall become like wool. (Isaiah 1:18)
Hebrews explains how it is that Christ’s priestly work brings this cleansing.
11But when Christ appeared as a high priest of the good things that have come, then through the greater and more perfect tent ( not made with hands, that is, not of this creation) 12he entered once for all into the holy places, not by means of the blood of goats and calves but by means of his own blood, thus securing an eternal redemption. 13For if the blood of goats and bulls, and the sprinkling of defiled persons with the ashes of a heifer, sanctify for the purification of the flesh, 14how much more will the blood of Christ, who through the eternal Spirit offered himself without blemish to God, purify our conscience from dead works to serve the living God. (Heb 9:11-14)
McCheyne’s Bible Reading Plan: Num 11, Ps 48, Is 1, Heb 9