Communion, also called the Lord's Supper in many Protestant denominations, or the Eucharist in the more liturgical churches, is one of the two sacraments specifically instituted
by Christ in the New Testament, the other being Baptism.
Jesus instituted this ordinance with his disciples at the Last Passover on the eve of his arrest. This event is specifically recorded in Luke 22:14-23 with Jesus saying "do this in remembrance of me." (22:19) The remembrance is to be of Christ's
person and work, specifically his sacrificial death on the cross. This ordinance is a sign of God's grace whereby:
- Jesus gave his body and blood for our sins (Luke 22:19-20),
- he entered into a covenant with us (Luke 22:20, 1 Cor 11:25), and
- he joined us into one body, his church (1 Cor 10:16).
The Westminster Confession defines it in this manner: "Our Lord Jesus, in the night wherein He was betrayed, instituted the sacrament of His body and blood, called the Lord's Supper,
to be observed in His Church, unto the end of the world, for the perpetual remembrance of the sacrifice of Himself in His death; the sealing all benefits thereof unto true believers, their spiritual nourishment and
growth in Him, their further engagement in and to all duties which they owe unto Him; and, to be a bond and pledge of their communion with Him, and with each other, as members of His mystical body." (WCF, 29.1).
Luther's position (particularly as developed by subsequent Lutheranism) is referred to as a "real physical presence" of Christ in the elements. The bread is still bread, but it is also truly the body of Christ.
And while the wine does not lose its "wine-ness", it is very much the actual blood of Christ. Luther found Jesus' words "This is my body" (Hoc est corpus meum) as a mandate for such an understanding. This view is often conflated
with consubstantiation, which is a philosophical rather than a theological view.
The position associated with Zwingli is sometimes referred to as the memorialist position, or the "real absence" view. That might be somewhat misleading: everyone agrees that the Supper is to be a memorial of Christ's death and resurrection.
The question is whether it is more than that. While most modern evangelicals suggest, at least in practice, that it is not - Zwingli does seem to have some place for the notion of a spiritual feeding of Christ. Ultimately though, both he and Oecolampadius
rejected Luther's position at Marburg because they saw it as a threat to the validity of Christ's resurrection and ascension - if Christ was physically resurrected in body, that body cannot be in two places at once (ie. at the right hand of
the Father and in the bread/wine). To argue that it could, as Luther did, seemed to challenge the physical nature of Christ's resurrection.
Calvin's understanding would emerge after that of Luther and Zwingli, but attempted to offer somewhat of a middle ground. He drew heavily on Augustinian definitions of the nature of sacraments, arguing that the Eucharist was
a visible sign of an invisible reality. So that while the bread is nothing more than bread, it signifies and presents to us a spiritual reality that takes place - a spiritual feeding on Christ by which believers are nourished. Calvin sought to answer
the question posed by Zwingli (on how Christ could be in two places at once) by proposing that at the Supper, Christians are taken into communion with Christ in heaven by the Holy Spirit. So Christ is exclusively present in body at the right hand of
the Father. If that is confusing to you, don't worry - Calvin himself admitted a level of mystery, saying that he would rather experience the reality of the Supper than understand it.