What were the differences between Martin Luther's concept of the Eucharist and that of the Roman Catholic Church?
One of the major ways in which Roman Catholicism differs from Lutheranism, and other forms of Protestantism, is in its practice and understanding of the Eucharist (mass, communion).
Luther intiated several reforms with respect to the Eucharist. In Romanism, communion is given in only one kind to the laity (i.e. the laity receive bread but not wine) and both kinds only to the clergy. In Protestantism, the laity as well as the clergy receive communion in both kinds.
Until the reforms of Vatican II in 1962, the mass was said in Latin, which was only understood by the educated clergy. Under Luther, mass was said in the vernacular, which was understood by the laity.
The priest faces the congregation during services, so that the congregation can hear him (or her) in Protestant churches, but faces towards an altar against the extreme east end of the church in the traditional tridentine mass of Romanism.
Roman Catholics believe that during the consecration, the bread and wine undergo transubstantiation, and are changed into the literal body and blood of Christ (in essence, not outward form). This makes possible reservation of the host which can later be carried to the sick who missed mass. According to Lutheranism, the communicants receive the body and blood of Christ spiritually as they consume the (unchanged) bread and wine, a position known as consubstantiation.