Charles Ellicott Commentary Isaiah 59:21

"And as for me, this is my covenant with them, saith Jehovah: my Spirit that is upon thee, and my words which I have put in thy mouth, shall not depart out of thy mouth, nor out of the mouth of thy seed, nor out of the mouth of thy seed`s seed, saith Jehovah, from henceforth and for ever." — Isaiah 59:21 (ASV)

As for me, this is my covenant ... —The words are, as to their form, an echo of Genesis 17:4; as to their meaning, the germ of Jeremiah 31:31; Hebrews 8:10; Hebrews 10:16. The new covenant is to involve the gift of the Spirit, that writes the law of God inwardly in the heart, as distinct from the Law, which is thought of as outside the conscience, doing its work as an accuser and a judge.