“Behold, the days are coming, declares the Lord, when I will make a new covenant with the house of Israel and the house of Judah . . . ”(Jeremiah 31:31)

God is just and holy and separated from sinners like us. This is our main problem at Christmas — and every other season. How shall we get right with a just and holy God?

Nevertheless, God is merciful and has promised in Jeremiah 31 (five hundred years before Christ) that someday he would do something new. He would replace shadows with the Reality of the Messiah. And he would powerfully move into our lives and write his will on our hearts so that we are not constrained from outside, but are willing from inside, to love him and trust him and follow him.
That would be the greatest salvation imaginable — if God should offer us the greatest Reality in the universe to enjoy and then move in us to know that Reality in such a way that we could enjoy it with the greatest freedom and the greatest pleasure possible. That would be a Christmas gift worth singing about.

That is, in fact, what he promised in the new covenant. But there was a huge obstacle. Our sin. Our separation from God because of our unrighteousness.

How shall a holy and just God treat us sinners with so much kindness as to give us the greatest Reality in the universe (his Son) to enjoy with the greatest possible joy?

The answer is that God put our sins on his Son, and judged them there, so that he could put them out of his mind, and deal with us mercifully and remain just and holy at the same time. Hebrews 9:28 says Christ was “offered once to bear the sins of many.”
Christ bore our sins in his own body when he died (1 Peter 2:24). He took our judgment (Romans 8:3). He canceled our guilt (Romans 8:1). And that means our sins are gone (Acts 10:43). They do not remain in God’s mind as a basis for condemnation. In that sense, he “forgets” them (Jeremiah 31:34). They are consumed in the death of Christ.

Which means that God is now free, in his justice, to lavish us with the all the unspeakably great new covenant promises. He gives us Christ, the greatest Reality in the universe, for our enjoyment. And he writes his own will — his own heart — on our hearts so that we can love Christ and trust Christ and follow Christ from the inside out, with freedom and joy.

— John Piper