In the following excerpt, Luther comments about Jacob's ten sons trying to
  cover up their sin of selling their brother Joseph into slavery (Gen. 42:34).
  However, unknown to them, it is Joseph in Egypt who is trying to expose their
  sin by requiring his brothers to bring Benjamin to him. Joseph planted a cup
  and their money, a reminder of their sin, in the sacks on their donkeys.
  Luther then compares this to God exposing all our sin at the cross of Christ.
  In one of the most brilliant observations about the crucifixion I've read,
  Luther concludes that Christ is the beast of burden who caries our sin. Luther
  makes the comparison as follows:
  
  "We see that the same thing happens in the household and in the state;
  for whatever sin is committed, this is thrown back on no one, because no one
  easily allows his evil deeds to be made known and censured. They are all
  pious; no one wants to admit that he has done wrong. Thus our vitiated and
  corrupted nature is also seen in actual sins. Who, then, has sinned? Who will
  make expiation? The Son of God alone is the sinner. No one else. He alone
  bears the sin and says (Ps. 51:1): 'Have mercy on Me, because I have sinned
  against Thee.' And again (Ps. 40:12): 'My iniquities have multiplied more than
  the hairs of My head.' He acknowledges and confesses sin in earnest, not for
  His own Person, which is holy and righteous, but as a Pleader and Advocate
  before the Judge; and He renders satisfaction for us.
  Therefore learn carefully the article about original sin. And you should
  not argue about why God has permitted what people with an inquisitive bent are
  accustomed to inquire into. No, you should rather ask how we are rescued and
  freed from this evil and know that God speaks with us to arouse us to
  acknowledge it. When it has been acknowledged, He says: 'Your sins are
  forgiven you; take heart, My son (cf. Matt. 9:2), because I have given My Son
  as a Lamb that is spotless from the beginning. If you acknowledge and confess
  your iniquity and transgressions, then My Son will be the propitiation for
  your sins; He will be your sanctification, redemption, righteousness, and
  wisdom, 1 Cor. 1:30.'
  Then Holy Scripture is open, and you will see that it teaches nothing else
  than the fact that God sets forth His Son, who was crucified, resurrected, and
  delivered for our sins. Yet this will not enter your heart, nor will you
  attain to the knowledge of what Christ is, unless you understand, with
  Scripture as your mentor, what you are before God. But you will learn this
  from Gen. 3:19, where it is taught that the punishment of death was inflicted
  on the whole human race on account of original sin, the fruits of which show
  how great its wickedness and perversity are. For what great rebellion and
  obstinacy there are in man's earliest years! What ragings and flames of lusts,
  hatreds, greed, and envy there are in youth and throughout life! Although we
  are born without actual sins, later, as time goes on, an infinite multitude of
  vices bursts forth. What is the source of these foul monsters?"
  The philosophers Socrates and Cicero do not know. But Scripture says that
  we are born as children of wrath (Eph. 2:3) and subject to original
  corruption; and the healthier, more learned, and wiser we are, the more pride
  and other sins we have. For nature is vitiated. But the papists interpose that
  it has been healed through Baptism and that sins have been forgiven. True
  enough; but, as Augustine says, they have not yet been removed. Because the
  papists do not know this, they never penetrate to the true light of Scripture,
  of the kingdom of Christ, and of theology as a whole.
  To be sure, the wounds of the half-dead man have been bound up, as the
  parable in Luke 10:34 states. Oil and wine have been poured on them, and the
  gift of the Holy Spirit has begun. Nevertheless, the wounds are still deadly.
  Care has been taken to heal him. But he has not yet been completely restored.
  If you should want to say that there is no wound, that there is no danger,
  find out whether a half-dead man can walk, work, and do what a healthy man can
  do. He is carried by the beast on which he has been placed. He does not work;
  he does not walk. Thus through Baptism we have been taken upon God's beast,
  that is, the most precious sacrifice for us, or the humanity of Christ, by
  which we are carried. Although we have been accepted once, yet we are cared
  for and healed from day to day." LW 7:280
  Note: Quotations are taken from the American Edition of
Luther's Works, jointly published by Fortress Press and Concordia Publishing
House.