Jeremiah 31:33 - The New Covenant.

I will put my law in their minds and write it on their hearts. I
will be their God, and they will be my people. Jer 31:33 (NIV)

The high point of Jeremiah's prophecies, this passage is the
longest sequence of OT verses to be quoted in its entirety in the NT.

The principal OT passage on the new covenant.  Its provisions
will include (1) a change of heart, (2) fellowship with God, (3)
knowledge of the Lord, and (4) forgiveness of sins. [Ryrie SB]

Put my law in their minds: Internally, in contrast to setting it
before them externally. [NIV SB]

Write it on their hearts: So that it effectively governs their
lives, in contrast to the ineffectiveness of merely presenting it in
writing. [NIV SB]

The difference between the two covenants is not in the behavior
expected under each, but in the differing relationship with God. Both
require obedience, but the old being external cannot be kept; the new
being internal can be a reality. [Passages Of Life SB]

The old covenant, broken by the people, would be replaced by a
new covenant. It offers a unique personal relationship with God
himself, with his laws written on individuals' hearts instead of on
stone. Jeremiah looked forward to the day when Jesus would come to
establish this covenant. But for us today, this covenant is here. We have
the wonderful opportunity to make a fresh start and establish a
permanent, personal relationship with God. [Life Application SB]

The same law that was engraved upon the tables of stone is
written by the Holy Spirit upon the tables of the heart. Instead of
going about to establish our own righteousness we accept the
righteousness of Christ. His blood atones for our sins. His obedience is
accepted for us. Then the heart renewed by the Holy Spirit will bring
forth "the fruits of the Spirit." Through the grace of Christ we shall
live in obedience to the law of God written upon our hearts. Having
the Spirit of Christ, we shall walk even as He walked. PP370-2