I started rereading "The Lord's Supper" By Martin Chemnitz, chapter by chapter as my devotional book. I like to insert some devotional reading between the Apostle's Creed, and the Lord's Prayer, during the rite of morning prayer found in the back of the Concordia ESV Bibles. If you don't know this book, it is one well worth becoming acquainted with. Martin Chemnitz gives the Lutheran doctrine of the Lord's Supper, a thourough systematic, and enlightning treatment. It is polemical, meaning he takes the alternate views and probes their weaknesses, and is not at all ashamed of what he believes as a Lutheran concerning the Lord's Supper.
In the fifth Chapter he proceeds to talk about 3 different types of eating, positing a Sacramental eating which opponents of the Lutheran, Biblical doctrine, of the Lord's Supper, do not discern. Most will only recognize a physical eating, and a Spiritual eating.
The physical eating is what we do with a steak, medium rare, and hot off the grill on Friday night. The cow is masticated, swallowed, some of it nourishes our body, and the rest is excremented. Chemnitz points out that this is what happens with the bread in the Lord's supper, but not the body of Christ. The idea that we eat the body of Christ in this manner is often refferred to as a Capernaitic eating, attributing it to the misunderstanding of the crowds in John 6.
Spiritual eating is that of faith ascending into the highest heavens and embracing Christ in all his majesty. Chemnitz points out that this must be done in the supper, if the supper is to be salutary, but if this is all that is done, then the Lord's Supper could be celebrated without any external reception of the mouth. This is true because faith is allways embracing Christ in his majesty.
Sacramental eating occupies the center of the controversy. Chemnitz maintains that this is the way in which we eat the body of Christ, when we receive the bread with our mouths. Christ's true body is given to us with the bread, and consumed sacramentally. It is, if you will, a supernatural, or miraculous consumption. But this does not require faith. All who receive the bread, believers or not, recieve the body of Christ in this sacramental reception. For Jesus clearly calls the bread his body, we can not believe therefore, that all do not actually receive his body. Otherwise Paul would not warn us of eating and drinking the body and blood to our judgement. In eating Christ, we eat God, and that can be dangerous for the unrepentant, or the unbelievers. It would also be casting pearls before the swine. Yet in faith, the benefits Christ won for us on the cross are sealed to us, and the body of Christ becomes a heavenly and spiritual nourishment for both BODY AND SOUL of believers unto eternal life.
He ends saying "Therefore, just as there is one thing whichis sealed and another thing by which the sealing is done, so also the spiritual eating of Christ which is sealed is one thing, and the the sacramental eating of the body of Christ by which the sealing is done is another."