I have put My law within you and have written it on your heart.

Jeremiah 31:33 New International Version (NIV)

33 “This is the covenant I will make with the people of Israel
    after that time,” declares the Lord.
“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.

In this promise, the Lord tells Israel that a day is coming where He will place His very law in their inward parts and write it on their hearts. No longer will they struggle to walk in God's plan and purpose for them because His plan and purpose will be inside of them.

When Jesus died and was buried and rose from the dead, He brought in a completely new covenant where He exchanged our life for His life. Everything that Jesus had, became ours. The law of our God has now been written on our hearts because our old life has died and our new life is hid with Christ in God. 

Because of Jesus, His God has become our God, and we have become His people. What a glorious promise and the fulfillment of the new covenant that God spoke of in Jeremiah's day. We are not separate from God and His ways. He lives in us and through us by the power of His Holy Spirit. We can rest in this amazing truth. He has written His law on our hearts!