Luther on the Sacraments

Summarizing Martin Luther's view on the blessed Eucharist, Gritsch and Jenson write, "The Lord's Supper is the combat ration of the church struggling to do God's will in the world. "If you could see how many daggers, spears, and arrows are at every moment aimed at you, you would be glad to come to the sacrament as often as possible (LC V, 82). Even children should partake of the Lord's Supper, "for they must help us to believe, to love, to pray, and to fight the devil (LC V, 87). In the sacrament of the altar, Christians not only remember Christ's sacrifice, his historicity, and anticipate his victorious return and his futurity, but also enjoy his 'real presence' in bread and wine." Lutheranism: The Theological Movement and Its Confessional Writings, pp. 75-76

A bit later there is a very helpful summary of the main differences between the Lutheran and the reformed approach to the Supper:

Calvinists taught that Christ comes spiritually to the soul by the word of the gospel. Within the gospel-event, the bread and wine have no role christologically different from the role of other, audible words; they are 'sumbols,' material adaptations of the gospel message to the material conditions of our perception. Christ's spiritual presence does not necessarily involve his bodily presence; the latter remains, as is proper to a body, at one place at one time, now 'with the Father' to whom he ascended. Our communion with Christ can nevertheless be bodily, because of the space and time-transcending character of spiritual realities; stirred to faith by Christ's spiritual presence in the words and symbols, our souls are carried to God, there to receive the blessing also of his bodily self-giving. Since the bread and wine, as symbols, mean the body and blood of the Lord, and since the faith in which that meaning terminates is indeed a reception also of his true bodily presence, the bread and wine can truly be said to 'be' his body and blood.

So far so good, apparently: the Reformation point is made without radical conflict with the philosophical tradition. But according to this teaching, Christ gives himself into bodily communion only with those who in fact are moved to faith by the symbolic bread and wine. The Lutherans objected that this does the same thing to the visible word that medieval doctrine did to the audible word: if these propositions enter the reflection that is internal to someone's experience of the sacrament, the gift of the sacrament must become conditional for him. To know if the gift is there, I must know if I believe it is there - which makes faith a condition and a work, and must also generate an endless  dialectic. The 'eating by the unbelievers,' affirmed by the Lutherans and denied by the Calvinists, became and remained the main point by which the two movements divided.