Jamieson, Fausset and Brown on Hebrews 8:12

12. For, &c.--the third of "the better promises" (He 8:6). The forgiveness of sins is, and will be, the root of this new state of inward grace and knowledge of the Lord. Sin being abolished, sinners obtain grace.

I will be merciful--Greek, "propitious"; the Hebrew, "salach," is always used of God only in relation to men.

and their iniquities--not found in Vulgate, Syriac, Coptic, and one oldest Greek manuscript; but most oldest manuscripts have the words (compare He 10:17).

remember no more--Contrast the law, He 10:3.