Question: What doctrine did Luther and Zwingli disagree on and what was their disagreement?
About the article "Protestant Reformation", section "Zwingli".

Candidate passages:
Beginning in 1524, Bucer concentrated on the main issue dividing leading reformers, the eucharist.
In this dispute, he attempted to mediate between Martin Luther and Huldrych Zwingli.
The two theologians disagreed on whether the body and blood of Christ were truly present within the elements of bread and wine during the celebration of the Lord's Supper.
Luther believed in the real presence of Christ; and Zwingli believed in a symbolic presence.
By late 1524, Bucer had abandoned the idea of the real presence and, after some exegetical studies, accepted Zwingli's interpretation.
However, he did not believe the Reformation depended on either position but on faith in Christ, other matters being secondary.
In this respect he differed from Zwingli.
In March 1526, Bucer published "Apologia", defending his views.
He proposed a formula that he hoped would satisfy both sides: different understandings of scripture were acceptable, and church unity was assured so long as both sides had a "child-like faith in God".
Bucer stated that his and Zwingli's interpretation on the eucharist was the correct one, but while he considered the Wittenberg theologians to be in error, he accepted them as brethren as they agreed on the fundamentals of faith.
He also published two translations of works by Luther and Johannes Bugenhagen, interpolating his own interpretation of the Lord's Supper into the text.
This outraged the Wittenberg theologians and damaged their relations with Bucer.
In 1528, when Luther published , detailing Luther's concept of the sacramental union, Bucer responded with a treatise of his own, .
It took the form of a dialogue between two merchants, one from Nuremberg who supported Luther and the other from Strasbourg who supported Bucer, with the latter winning over his opponent.
Bucer noted that as Luther had rejected impanation, the idea that Christ was "made into bread", there was no disagreement between Luther and Zwingli; both believed in a spiritual presence of Christ in the eucharist.
Luther harshly rejected Bucer's interpretation.
During this time, Bucer and Zwingli remained in close touch, discussing other aspects of theology and practice such as the use of religious images and the liturgy.
Bucer did not hesitate to disagree with Zwingli on occasion, although unity between Strasbourg and the Swiss churches took priority over such differences.
In 1527, Bucer and Capito attended a disputation in Bern to decide whether the city should accept reformed doctrines and practices.
Bucer provided strong support for Zwingli's leading role in the disputation, which finally brought the Reformation to Bern.
The last meeting between Zwingli and Luther was at the Marburg Colloquy of October 1529, organised by Philip of Hesse and attended by various leading reformers, including Bucer.
Luther and Zwingli agreed on 13 of the 14 topics discussed, but Zwingli did not accept the doctrine of the real presence, on which Luther would not compromise.
After the discussion broke down between the two, Bucer tried to salvage the situation, but Luther noted, "It is obvious that we do not have one and the same spirit."
The meeting ended in failure.
The following year, Bucer wrote of his disappointment at doctrinal inflexibility: 
The Eucharist was a key centre of controversy in the Reformation as it not only focused differences between the reformers and the church but also between the reformers themselves.
For Zwingli it was matter of attacking a doctrine that imperiled the understanding and reception of God's gift of salvation, while for Luther it was a matter of defending a doctrine that embodied that gift.
It is not known what was Zwingli's eucharistic theology before he became a reformer and there is disagreement among scholars about his views during his first few years as a priest.
In the eighteenth article of "The Sixty-seven Articles" (1523) which concerns the sacrifice of the mass, he states that it is a memorial of the sacrifice.
He expounds on this in "An Exposition of the Articles" (1523).
Zwingli credited the Dutch humanist, Cornelius Henrici Hoen (Honius), for first suggesting the "is" in the institution words "This is my body" meant "signifies".
Hoen sent a letter to Zwingli in 1524 with this interpretation along with biblical examples to support it.
It is impossible to say how the letter impacted Zwingli's theology although Zwingli claimed that he already held the symbolic view when he read the letter.
He first mentioned the "signifies" interpretation in a letter to Matthäus Alber, an associate of Luther.
Zwingli denies transubstantiation using John 6:63, "It is the Spirit who gives life, the flesh is of no avail", as support.
He commended Andreas Karlstadt's understanding of the significance of faith, but rejected Karlstadt's view that the word "this" refers to Christ's body rather than the bread.
Using other biblical passages and patristic sources, he defended the "signifies" interpretation.
In "The Eucharist" (1525), following the introduction of his communion liturgy, he laid out the details of his theology where he argues against the view that the bread and wine become the body and blood of Christ and that they are eaten bodily.
The conflict between Zwingli and Luther began in 1525, but it was not until 1527 that Zwingli engaged directly with Luther.
The culmination of the controversy was the Marburg Colloquy in 1529.
He wrote four responses leading up to the meeting: "A Friendly Exegesis" (1527), "A Friendly Answer" (1527), "Zwingli's Christian Reply" (1527), and "Two Replies to Luther's Book" (1528).
They examined Luther's point-of-view rather than systematically presenting Zwingli's own.
Some of his comments were sharp and critical, although they were never as harsh and dismissive as some of Luther's on him.
However, Zwingli also called Luther "one of the first champions of the Gospel", a David against Goliath, a Hercules who slew the Roman boar.
Martin Bucer and Johannes Oecolampadius most likely influenced Zwingli as they were concerned with reconciliation of the eucharistic views.
The main issue for Zwingli is that Luther puts "the chief point of salvation in the bodily eating of the body of Christ".
Luther saw the action as strengthening faith and remitting sins.
This, however, conflicted with Zwingli's view of faith.
The bodily presence of Christ could not produce faith as faith is from God, for those whom God has chosen.
Zwingli also appealed to several passages of scripture with John 6:63 in particular.
He saw Luther's view as denying Christ's humanity and asserted that Christ's body is only at one place and that is at the right hand of God.
The Marburg Colloquy did not produce anything new in the debate between the two reformers.
Neither changed their positions, but it did produce some further developments in their own views.
Zwingli, for example, noted that the bread was not mere bread and affirmed terms such as "presence", "true", and "sacramental".
However, it was Zwingli's and Luther's differences in their understanding of faith, their Christology, their approach and use of scripture that ultimately made any agreement impossible.
Near the end of his life Zwingli summarized his understanding of the Eucharist in a confession sent to King Francis I, saying:"We believe that Christ is truly present in the Lord’s Supper; yea, we believe that there is no communion without the presence of Christ.
This is the proof: 'Where two or three are gathered together in my name, there am I in the midst of them' (Matt.
18:20).
How much more is he present where the whole congregation is assembled to his honor!
But that his body is literally eaten is far from the truth and the nature of faith.
It is contrary to the truth, because he himself says: 'I am no more in the world' (John 17:11), and 'The flesh profiteth nothing' (John 6:63), that is to eat, as the Jews then believed and the Papists still believe.
It is contrary to the nature of faith (I mean the holy and true faith), because faith embraces love, fear of God, and reverence, which abhor such carnal and gross eating, as much as any one would shrink from eating his beloved son.… We believe that the true body of Christ is eaten in the communion in a sacramental and spiritual manner by the religious, believing, and pious heart (as also St. Chrysostom taught).
And this is in brief the substance of what we maintain in this controversy, and what not we, but the truth itself teaches."
Anabaptism in Switzerland began as an offshoot of the church reforms instigated by Ulrich Zwingli.
As early as 1522 it became evident that Zwingli was on a path of reform preaching when he began to question or criticize such Catholic practices as tithes, the mass, and even infant baptism.
Zwingli had gathered a group of reform-minded men around him, with whom he studied Classic literature and the Scriptures.
However, some of these young men began to feel that Zwingli was not moving fast enough in his reform.
The division between Zwingli and his more radical disciples became apparent in an October, 1523 disputation held in Zurich.
When the discussion of the mass was about to be ended without making any actual change in practice, Conrad Grebel stood up and asked "what should be done about the mass?"
Zwingli responded by saying the Council would make that decision.
At this point, Simon Stumpf, a radical priest from Hongg, answered saying, "The decision has already been made by the Spirit of God.
"This incident illustrated clearly that Zwingli and his more radical disciples had different expectations.
To Zwingli, the reforms would only go as fast as the city Council allowed them.
To the radicals, the council had no right to make that decision, but rather the Bible was the final authority of church reform.
Feeling frustrated, some of them began to meet on their own for Bible study.
As early as 1523, William Reublin began to preach against infant baptism in villages surrounding Zurich, encouraging parents to not baptize their children.
Seeking fellowship with other reform-minded people, the radical group wrote letters to Martin Luther, Andreas Karlstadt, and Thomas Müntzer.
Felix Manz began to publish some of Karlstadt's writings in Zurich in late 1524.
By this time the question of infant baptism had become agitated and the Zurich council had instructed Zwingli to meet weekly with those who rejected infant baptism "until the matter could be resolved."
Zwingli broke off the meetings after two sessions, and Felix Manz petitioned the Council to find a solution, since he felt Zwingli was too hard to work with.
The council then called a meeting for January 17, 1525.
The Council ruled in this meeting that all who continued to refuse to baptize their infants should be expelled from Zurich if they did not have them baptized within one week.
Since Conrad Grebel had refused to baptize his daughter Rachel, born on January 5, 1525, the Council decision was extremely personal to him and others who had not baptized their children.
Thus, when sixteen of the radicals met on Saturday evening, January 21, 1525, the situation seemed particularly dark.
The Hutterian Chronicle records the event:After Blaurock was baptized, he in turn baptized others at the meeting.
Even though some had rejected infant baptism before this date, these baptisms marked the first re-baptisms of those who had been baptized as infants and thus, technically, Swiss Anabaptism was born on that day. 
While Zwingli carried on the political work of the Swiss Reformation, he developed his theological views with his colleagues.
The famous disagreement between Luther and Zwingli on the interpretation of the eucharist originated when Andreas Karlstadt, Luther's former colleague from Wittenberg, published three pamphlets on the Lord's Supper in which Karlstadt rejected the idea of a real presence in the elements.
These pamphlets, published in Basel in 1524, received the approval of Oecolampadius and Zwingli.
Luther rejected Karlstadt's arguments and considered Zwingli primarily to be a partisan of Karlstadt.
Zwingli began to express his thoughts on the eucharist in several publications including "de Eucharistia" (On the Eucharist).
He attacked the idea of the real presence and argued that the word "is" in the words of the institution—"This is my body, this is my blood"—means "signifies".
Hence, the words are understood as a metaphor and Zwingli claimed that there was no real presence during the eucharist.
In effect, the meal was symbolic of the Last Supper.
By spring 1527, Luther reacted strongly to Zwingli's views in the treatise "Dass Diese Worte Christi "Das ist mein Leib etc."
noch fest stehen wider die Schwarmgeister" (That These Words of Christ "This is My Body etc."
Still Stand Firm Against the Fanatics).
The controversy continued until 1528 when efforts to build bridges between the Lutheran and the Zwinglian views began.
Martin Bucer tried to mediate while Philip of Hesse, who wanted to form a political coalition of all Protestant forces, invited the two parties to Marburg to discuss their differences.
This event became known as the Marburg Colloquy.
Zwingli accepted Philip's invitation fully believing that he would be able to convince Luther.
By contrast, Luther did not expect anything to come out of the meeting and had to be urged by Philip to attend.
Zwingli, accompanied by Oecolampadius, arrived on 28 September 1529 with Luther and Philipp Melanchthon arriving shortly thereafter.
Other theologians also participated including Martin Bucer, Andreas Osiander, Johannes Brenz, and Justus Jonas.
The debates were held from 1–3 October and the results were published in the fifteen "Marburg Articles".
The participants were able to agree on fourteen of the articles, but the fifteenth article established the differences in their views on the presence of Christ in the eucharist.
Afterwards, each side was convinced that they were the victors, but in fact the controversy was not resolved and the final result was the formation of two different Protestant confessions. 
Zwingli responded by writing the "Friendly Rejoinder and Rebuttal to the Sermon of the Eminent Martin Luther against the Fanatics."
In his "Friendly Exposition", written at the same time, Zwingli found "The Sacrament of the Body and Blood of Christ" to be "forgettable".
Between these two works, Zwingli concluded that he had destroyed Luther's singular authority and replaced Luther's view on the Sacrament of the Altar with the correct one.
Zwingli wrote that he was not placing reason over the Scriptures, but instead properly distinguishing between the human and divine natures of Christ.
Since Christ's human nature could not participate in the omnipresent attribute of the divine nature, his body and blood could not be truly present in the bread and wine.
Luther's characterization of him as placing reason over Scripture was therefore a straw-man argument.
Luther's custom of referring to those who opposed him in non-Christian terms in "The Sacrament of the Body and Blood of Christ" sounds unusual to modern ears and therefore merits discussion.
However strange-sounding today, this practice was consistent with what evangelicals at this early stage in the Reformation believed about the Reformation and the church.
From 1520 and continuing through 1529, evangelicals thought the Reformation would unite rather than divide the church.
Jews and misled Christians under Rome would convert to the true faith, while everyone else was a false Christian.
In Zwingli's view, Luther was only a Christian as any other and his talents as a theologian did not give him the right to make tyrannical judgments, for example, terming him a heretic, fanatic, and sacramentarian.
Zwingli also rejected Luther's claim that the Swiss Reformers relied upon him in their preaching about Christ's death, claiming instead to follow Augustine and Erasmus.
Although he admitted that Luther began the Reformation, he claimed that Luther was actually further behind him in the work of reforming.
He considered Luther unscholarly and demanded that he retract his published views on the Lord's Supper going back 1519.
Instead of condemning Luther, Zwingli found him in need of brotherly prayer.