Luther On Penal Substitution

Luther, commenting on Galatians 3:13, "Christ redeemed us from the curse of the law, having become a curse for us," states penal substitution like this:

We are sinners and thieves, and therefore guilty of death and everlasting damnation. But Christ took all our sins upon him, and for them died upon the cross . . . all the prophets did foresee in spirit, that Christ should become the greatest transgressor, murderer, adulterer, thief, rebel, blasphemer, etc. that ever was for he being made a sacrifice, for the sins of the whole world, is now an innocent person and without sins . . . our most merciful Father, seeing us to be oppressed overwhelmed with the curse of the law, and so to be holden under the same that we could never be delivered from it by our own power, sent his only Son into the world and laid upon him all the sins of all men, saying: Be thou Peter that denier; Paul that persecutor, blasphemer and cruel oppressor; David that adulterer; that sinner which did eat the apple in Paradise; that thief which hanged upon the cross; and, briefly, be thou the person which hath committed the sins of all men; see therefore that thou pay and satisfy for them. Here now cometh the law and saith: I find him a sinner, and that such a one as hath taken upon him the sins of all men, and I see no sins but in him; therefore let him die upon the cross. And so he setteth upon him and killeth him. By this means the whole world is purged and cleansed from all sins, and so delivered from death and all evils.

From Luther's Commentary on Galatians