From Martyn Lloyd Jones:
The cross is a mightly declaration. And what it says is this: The Son is a propitiation. In other words, God on the cross was punishing sin. He said that He would, and He has done it.... It is a mightly declaration that God has done what He has always said He would do--namely, that He would punish sin, and the wages of sin is death. And there you see it happening upon the cross. It is an annoucement, a proclamation, that this is God's way of dealing with the problem of sin. --Walking With God Day by Day, April 19th reading.