16. In this light I now remind you that these words are not to be misconstrued and made to refer to the Sacrament of the Altar; whoever so interprets them does violence to this Gospel text. There is not a letter in it that refers to the Lord's Supper. Why should Christ here have in mind that Sacrament when it was not yet instituted? The whole chapter from which this Gospel is taken speaks of nothing but the spiritual food, namely, faith. When the people followed the Lord merely hoping again to eat and drink, as the Lord himself charges them with doing, he took the figure from the temporal food they sought, and speaks throughout the entire chapter of a spiritual food. He says: "The words that I have spoken unto you are spirit, and are life." Thereby he shows that he feeds them with the object of inducing them to believe on him, and that as they partook of the temporal food, so should they also partake of the spiritual. On this subject we will say more at some other time. [source]As we let Luther be Luther, the sermon goes on to say,
19. The whole New Testament treats of this spiritual supper, and especially does John here. The Sacrament of the Altar is a testament and confirmation of this true supper, with which we should strengthen our faith and be assured that this body and this blood, which we receive in the Sacrament has rescued us from sin and death, the devil, hell and all misery. Concerning this I have spoken and written more on other occasions.
The Steadfast Lutherans have an interesting article which refers to Luther's view. The article states,
There is a reason no orthodox Lutheran theologian saw John 6 as sacramental before the acceptance of historical criticism in the 1800’s. What is the reason behind this powerful fact? They read Scripture differently.What they mean by that is the narrative of John 6 was held to be an historical account of what Jesus said. Later historical criticism saw the text as "post-resurrection, interpretive, theological commentaries." So, when Luther's sermon asks, "Why should Christ here have in mind that Sacrament when it was not yet instituted?" The sermon sees John 6 as an historical account, not an interpretation of the early church community. The article goes on to say of Luther's view:
The basic argument is that the context of John 6 is the early church, not the words or history given in the text. Luther went to the text and saw an accurate reporting of Jesus’ dialogue with unbelieving Jews who merely wanted bread from Jesus.-snip-
Luther’s chronological argument (that the Supper was not yet instituted) assumed that John 6 was reliable history in every sense.