Quoting from E. G. Schwiebert's book Luther and His Times: The Reformation from a New Perspective:
In his tract [Concerning the Lord’s Supper, a Confession] of 1528 Luther wrote on this relationship:If you can say: Here is God, then you must also grant that the Man Christ is likewise present. And, if you were to point to a place where God is present and not the man, then the person would already be divided, for then I could really make the assertion: Here is God who is not man and never was man and not my God. …No, my dear fellow, where you place God, there you must likewise put the humanity of Christ, for they cannot be separated in that they have become one person.God is not limited to space, yet He revealed Himself in space. He can be where he wishes and can be everywhere present. To define this presence, Luther used the terms in, with, and under, implying “real presence,” yet not in the physical sense of space. The same Christ who could pass through stone walls, bolted doors, and disappear in a moment, could for Luther also be in the bread and wine through this quality of ubiquity, even though Communion was celebrated in ten thousand places at the same time.
E. G. Schwiebert, Luther and His Times: The Reformation from a New Perspective, (St. Louis: CPH, 1950) 703.