Perfect wrath

How the cross proclaims the law as well as the gospel:

Yea, what more forcible, more terrible declaration and preaching of God’s wrath against sin is there than just the suffering and death of Christ, His Son? But as long as all this preaches God’s wrath and terrifies men, it is not yet the preaching of the Gospel nor Christ’s own preaching, but that of Moses and the Law against the impenitent. For the Gospel and Christ were never ordained and given for the purpose of terrifying and condemning, but of comforting and cheering those who are terrified and timid. And again: Christ says, John 16:8: "The Holy Ghost will reprove the world of sin"; which cannot be done except through the explanation of the Law.
Formula of Concord, Solid Declaration 5:12