"Our basic arguments, on which we have always stood in this matter since the outbreak of the dispute regarding this article, are the same as those that Dr. Luther set down in the following words initially against the sacramentarians. (Dr. Luther in his Large Confession concerning the Holy Supper): My reasons upon which I rest in this matter are the following:
First, the comprehensible, bodily mode, as He went about bodily upon earth, when, according to His size, He vacated and occupied space [was circumscribed by a fixed place]. This mode He can still use whenever He will, as He did after the resurrection, and will use at the last day, as Paul says, 1 Tim. 6:15: "Which in His times He shall show, who is the blessed God [and only Potentate, the King of kings and Lord of lords]." And to the Colossians, 3:4: "When Christ, who is our Life, shall appear." In this manner He is not in God or with the Father, neither in heaven, as the mad spirits dream; for God is not a bodily space or place. And this is what the passages how Christ leaves the world and goes to the Father refer to which the false spirits cite.
Secondly, the incomprehensible, spiritual mode, according to which He neither occupies nor vacates space, but penetrates all creatures wherever He pleases [according to His most free will]; as, to make an imperfect comparison, my sight penetrates and is in air, light, or water, and does not occupy or vacate space; as a sound or tone penetrates and is in air or water or board and wall, and also does not occupy or vacate space; likewise, as light and heat penetrate and are in air, water, glass, crystal, and the like, and also do not vacate or occupy space; and much more of the like [many comparisons of this matter could be adduced]. This mode He used when He rose from the closed [and sealed] sepulcher, and passed through the closed door [to His disciples], and in the bread and wine in the Holy Supper, and, as it is believed, when He was born of His mother [the most holy Virgin Mary].
Thirdly, the divine, heavenly mode, since He is one person with God, according to which, of course, all creatures must be far more penetrable and present to Him than they are according to the second mode. For if, according to that second mode, He can be in and with creatures in such a manner that they do not feel, touch, circumscribe, or comprehend Him, how much more wonderfully will He be in all creatures according to this sublime third mode, so that they do not circumscribe nor comprehend Him, but rather that He has them present before Himself, circumscribes and comprehends them! For you must place this being of Christ, who is one person with God [for you must place this mode of presence of Christ which He has by His personal union with God], very far, far outside of the creatures, as far as God is outside of them; and again as deep and near within all creatures as God is within them. For He is one inseparable person with God; where God is, there must He also be, or our faith is false. But who will say or think how this occurs? We know indeed that it is so, that He is in God outside of all creatures, and one person with God, but how it occurs we do not know; it [this mystery] is above nature and reason, even above the reason of all the angels in heaven; it is understood and known only by God. Now, since it is unknown to us, and yet true, we should not deny His words before we know how to prove to a certainty that the body of Christ can by no means be where God is, and that this mode of being [presence] is false. This the fanatics must prove; but they will forego it.
Now, whether God has and knows still more modes in which Christ's body is anywhere, I did not intend to deny herewith, but to indicate what awkward dolts our fanatics are, that they concede to the body of Christ no more than the first, comprehensible mode; although they cannot even prove that to be conflicting with our meaning. For in no way will I deny that the power of God may accomplish this much that a body might be in many places at the same time, even in a bodily, comprehensible way. For who will prove that this is impossible with God? Who has seen an end to His power? The fanatics indeed think thus: God cannot do it. But who will believe their thinking? With what do they make such thinking sure?" (Article VII: The Holy Supper, paragraphs 93-103).
I certainly don't pretend to understand all of this, but at least it's an explanation of a problem that has been vexing me for some time now.
Also, how wonderful it is that the Book of Concord may be read in its entirety on the internet! http://bookofconcord.org/