Jesus makes the perfect Host.

A recent classroom interchange at Catholic university, in a history class on American Catholicism: Student: Interesting you should bring up Communion today. I just had a long argument with my parents about it. Professor: What was the argument? Student: They said the host becomes the actual Body of Christ during Mass. I told them they were mistaken, that Jesus' spirit is present in the congregation, and the Communion host is a symbol of that presence. Professor: Now what do you think? Student: From what you said in the lecture, my parents were right.

Right they are, now and forever. The church's teaching on the Eucharist is one of those infallible, irreformable dogmas whose history reveals a constant witness to a truth at the very heart of the Roman Catholic faith.

In the 1520s, only a few years after the Augustinian monk Martin Luther inaugurated the Protestant Reformation by nailing his complaints against the abuse of indulgences (the famous "95 Theses") to the door of the Wittenberg Castle Church, a dispute arose among the leading Protestant reformers regarding the doctrine of the Real Presence of Christ in the Eucharist.

The Roman Catholic Church first defined the Real Presence as dogma in 1079 when it proclaimed that after Consecration, the bread and wine "are the true body of Christ which was born of the Virgin Mary and hung on the cross for the salvation of the world and which now sits at the right hand of the Father, and the true blood of Christ which was shed from His side, not only as a sign and by virtue of the sacrament, but in their proper nature and true substance."

How is this miracle to be understood? If this is your question, you share with generations of medieval theologians the refusal to recognize that the words miracle and understand are incompatible. Their faith, to put it nobly, sought greater understanding.

Thus, in 1215, the Fourth Lateran Council in Rome introduced the concepts of species, substances, and transubstantiation to "explain" the dogma of the Real Presence.

For half a century this rather cryptic development inspired more questions than answers. To the rescue, eventually, rode the greatest of the scholastic theologians, Saint Thomas Aquinas, who defined substance as the invisible essence of a thing: that which makes it what it is. A thing's nonessential characteristics he referred to as accidents.

Aquinas's theology of transubstantiation held that the substance of the bread and wine is transformed by the priest into the substance of Christ's Body and Blood, while the accidents of bread and wine--their texture, shape, color--remain. (That's why the elements still taste and look like bread and wine rather than flesh and blood.) As he sometimes did, Aquinas borrowed the central concepts of this eucharistic theology from Aristotle--in this case, from the pagan philosopher's categories of substance and accidents.

By the time of the Reformation, Catholics held that what we now call the eucharistic prayer is a reenactment of Christ's self-sacrifice on the cross. The bread and wine consecrated in that prayer convey Christ's own life to the baptized. Christ's Body and Blood is fully and truly present in the bread and wine of the Eucharist. We become one with Christ's Body by partaking of it.

The Protestant reformers, in breaking away from the Roman church, knew that they had to revise this teaching, but they disagreed on how to do so. The Marburg Colloquy of 1529, a gathering of the major Saxon and Swiss theologians of the Reformation, was held to reach a consensus. At the gathering, Ulrich Zwingli, the founder of Protestantism in German Switzerland and the first Reformed theologian, argued that the Eucharist is not a reenactment of Christ's sacrifice but a memorial of that sacrifice.

The Last Supper and Calvary happened once, in the past, and that event alone was sufficient for our salvation, said Zwingli. In his view Roman Catholics, as usual, had perpetuated a dubious ritual to justify their own priestly existence and their unscriptural doctrine of the church.

Others at the colloquy warned their fellow Protestants that emphasizing the Real Presence of the Body and Blood and the words of Consecration would undermine the priority of faith over works, make the church--rather than Christ--the agent of salvation, and encourage precisely the kind of superstition the Reformation sought to end.

Martin Luther, however, dramatically defended the Real Presence against these interpretations. Pounding on the table, he repeated Christ's words at the Last Supper: "This is my body." For his colleagues hard of hearing, he wrote those words on the tablecloth, which immediately became a collector's item for Lutherans (and for Reformation historians). This dramatic gesture did not settle the matter, however. When Luther pointed to Christ's unambiguous words of commission in scripture, Zwingli responded, "Yes--'Do this in memory of me.'"

The Marburg Colloquy participants, while agreeing to disagree on the doctrine of the Real Presence, did manage to vote unanimously to reject the Roman Catholic understanding of the Mass as a reenactment of the sacrifice of Christ on the cross. For the reformers, no church should or would have a monopoly on the saving grace of the atonement.

Several fundamental articles of the Christian faith were at issue in this debate over the Real Presence:

1. The theological meaning of the Incarnation. We must receive Christ himself--Christ with his flesh--Luther insisted, because it was in his flesh that Christ redeemed us: "We do not meet God outside of the man Jesus Christ, in whom the whole fullness of God dwells."

Catholic bishops and the pope would have supported Luther on this point, had they not been annoyed with so many other things he said about them.

2. The paradox of redemption. Jesus redeemed us from sin and death at Calvary, but we must live that redeemed life--make it our own--by participating as members of Christ's mystical body. That means literally partaking of Christ's Body and Blood in the Eucharist, as well as putting our own bodies and blood at risk for our neighbors in need, as Jesus did.

Like Catholics, Luther distinguished the event of salvation, which occurred at a precise moment in history, from the moments when Christ offers himself to each person--in the sacraments, for example, or in prayer or in a moment of sudden realization (which some call a "born-again" experience). In those moments God's staggering generosity--embodied in the self-gift of Christ on the cross--is the basis for a new relationship with God that brings us eternal life.

To various degrees other reformers, such as Zwingli and later Calvin, rejected the Catholic insistence that people encounter Christ's saving presence exclusively, or even primarily, in the Eucharist or in the other sacraments.

3. The place of scripture in the theology of the Eucharist. Here the Protestants disagreed radically among themselves on what the New Testament teaches about the Eucharist and the Real Presence. Some could not square their reading of the role of the church, for example, with a doctrine that would make the repeated "sacrifice of the Mass" central to salvation.

And while Luther agreed with Catholics on the Real Presence, he made strong arguments against transubstantiation. He objected strenuously, for example, that Aristotle, a pagan philosopher, should be invoked to "explain" the greatest of Christian mysteries. Luther saw this as a clear-cut case of how the Roman church went astray when it tried to justify teachings not found in scripture by making a dubious appeal to "tradition."

Twenty years after the Marburg Colloquy, the Roman Catholic Church at the Council of Trent officially reconfirmed Aquinas's theology of the Real Presence. The term transubstantiation had a glorious run of more than 400 years. After Vatican II, some Roman Catholic theologians, partly as an ecumenical gesture, dropped the word and began to speak of transignification to convey the meaning of the Real Presence.

While the wording of its explanation has varied, however, the doctrine of the Real Presence remains bedrock, absolutely essential to the Christian faith of Roman Catholics. Someone ought to mention this to American Catholics, a majority of whom, according to recent polling data, believe that the Eucharist is a celebration of an event in the past, a kind of memorial of the Last Supper, and that Christ is present in the Eucharist only if the recipient believes him to be present. Zwingli would have been proud.

By R. Scott Appleby, a professor of history and director of the Cushwa Center for the Study of American Catholicism at the University of Notre Dame in Indiana.