It might be worth citing from the Smaller Catechism:
VI. The Sacrament of the Altar
As the head of the family should teach it in a simple way to his household.
What is the Sacrament of the Altar?
It is the true body and blood of our Lord Jesus Christ, under the bread and wine, for us Christians to eat and to drink, instituted by Christ Himself.
Where is this written?
The holy Evangelists, Matthew, Mark, Luke, and St. Paul, write thus:
Our Lord Jesus Christ, the same night in which He was betrayed, took bread: and when He had given thanks, He brake it, and gave it to His disciples, and said, Take, eat; this is My body, which is given for you. This do in remembrance of Me.
After the same manner also He took the cup, when He had supped, gave thanks, and gave it to them, saying, Take, drink ye all of it. This cup is the new testament in My blood, which is shed for you for the remission of sins. This do ye, as oft as ye drink it, in remembrance of Me.
What is the benefit of such eating and drinking?
That is shown us in these words: Given, and shed for you, for the remission of sins; namely, that in the Sacrament forgiveness of sins, life, and salvation are given us through these words. For where there is forgiveness of sins, there is also life and salvation.
How can bodily eating and drinking do such great things?
It is not the eating and drinking, indeed, that does them, but the words which stand here, namely: Given, and shed for you, for the remission of sins. Which words are, beside the bodily eating and drinking, as the chief thing in the Sacrament; and he that believes these words has what they say and express, namely, the forgiveness of sins.
Who, then, receives such Sacrament worthily?
Fasting and bodily preparation is, indeed, a fine outward training; but he is truly worthy and well prepared who has faith in these words: Given, and shed for you, for the remission of sins.But he that does not believe these words, or doubts, is unworthy and unfit; for the words 'For you' require altogether believing hearts.
Luther had made a strong attack on the Catholic understanding of the sacraments in The Babylonian Captivity of the Church. He was appalled by the pratice of communion in one kind, which he viewed as unscriptural. The doctrine of transubstantion was an absurd - an attempt to explain a mystery. To know that Christ is present - isn't that enough? Furthermore, the idea that the priest made a sacrifice on our behalf was equally unscriptural. Sacraments were for the nourishment and growth in faith of the people of God, not for accruing merit.
But Luther held onto the view that bread and wine really did become the body and blood of Christ. He did so not because it was traditional, but because it was (as he saw it) scriptural. This IS my body! The principle of the clarity of scripture seemed to be at stake here.
The controversy between the Reformers over this matter dominated the late 1520s. For Luther, the Word of God and the sacrament were inseparably linked. Both serve to mediate the power and presence of Christ. The sacraments, just like the word, could generate and support saving faith.
Furthermore, Luther asserted that Christ was 'ubiquitous': that is, the ascension of Christ 'to the right hand of God' did not mean that his presence was limited by space or time. Thus, he could be present in the sacrament in a saving way.
