The theology of Ulrich Zwingli was based on an interpretation of the Bible, taking scripture as the inspired word of God and placing its authority higher than what he saw as human sources such as the ecumenical councils and the Church Fathers. He also recognised the human element within the inspiration, noting the differences in the canonical gospels. Zwinglianism is the Reformed confession based on the Second Helvetic Confession promulgated by Zwingli's successor Heinrich Bullinger in the 1560s.

Zwingli's views on baptism were largely a response to Anabaptism, a movement which criticized the practice of infant baptism. He defended the baptism of children by describing it as a sign of a Christian's covenant with disciples and God just as God made a covenant with Abraham.

He denied the Catholic doctrine of transubstantiation and following Cornelius Henrici Hoen, he agreed that the bread and wine of the institution signify and do not literally become the body and blood of Jesus Christ. Zwingli's differences of opinion on this with Martin Luther resulted in the failure of the Marburg Colloquy to bring unity between the two Protestant leaders.

Zwingli believed that the state governed with divine sanction. He believed that both the church and the state are placed under the sovereign rule of God. Christians were obliged to obey the government, but civil disobedience was allowed if the authorities acted against the will of God. He described a preference for an aristocracy over monarchic or democratic rule.

Scripture The Bible is central in Zwingli's work as a reformer and is crucial in the development of his theology. Zwingli appealed to scripture constantly in his writings. This is strongly evident in his early writings such as Archeteles (1522) and The Clarity and Certainty of the Word of God (1522). He believed that man is a liar and only God is the truth. For him scripture, as God's word, brings light when there is only darkness of error.

Zwingli initially appealed to scripture against Catholic opponents in order to counter their appeal to the church—which included the councils, the Church Fathers, the schoolmen, and the popes. To him, these authorities were based on man and liable to error. He noted that "the fathers must yield to the word of God and not the word of God to the fathers".Huldreich Zwinglis Samtliche Werke, Vol. III, 505–509, as quoted in His insistence of using the word of God did not preclude him from using the councils or the Church Fathers in his arguments. He gave them no independent authority, but he used them to show that the views he held were not simply his own.

The inspiration of scripture, the concept that God or the Holy Spirit is the author, was taken for granted by Zwingli. His view of inspiration was not mechanical and he recognized the human element in his commentaries as he noted the differences in the canonical gospels. He did not recognize the apocryphal books as canonical. Like Martin Luther, Zwingli did not regard the Book of Revelation highly, and also did not accept a "canon within the canon", but he did accept scripture as a whole.

Soteriology

In the centuries leading up to the Reformation, an "Augustinian Renaissance" sparked renewed interest in the thought of Augustine of Hippo (354–430), who is widely regarded as the most influential patristic figure of the Reformation. Zwingli rooted his theology of salvation deeply in Augustinian soteriology alongside Martin Luther (1483–1546) and John Calvin (1509–1564). Augustine's theology was grounded in divine monergism, and implied a double predestination. Similarly, Zwingli's vision centered also on divine monergism. He affirmed that God predetermined both election to salvation and reprobation.

Baptism Zwingli's views on baptism are largely rooted in his conflict with the Anabaptists, a group whose beliefs included the rejection of infant baptism in favor of believer's baptism and centered on the leadership of Conrad Grebel and Felix Manz. In October 1523, the controversy over the issue broke out during the second Zürich disputation and Zwingli vigorously defended the need for infant baptism and his belief that rebaptism was unnecessary. His major works on the subject include Baptism, Rebaptism, and Infant Baptism (1525), A Reply to Hubmaier (1525), A Refutation (1527), and Questions Concerning the Sacrament of Baptism (1530).

In Baptism, Rebaptism, and Infant Baptism, Zwingli outlined his disagreements with both the Catholic and the Anabaptist positions. He accused the Anabaptists of adding to the word of God and noted that there is no law forbidding infant baptism. He challenged Catholics by denying that the water of baptism can have the power to wash away sin. Zwingli understood baptism to be a pledge or a promise, but he disputed the Anabaptist position that it is a pledge to live without sin, noting that such a pledge brings back the hypocrisy of legalism. He argued against their view that those that received the Spirit and were able to live without sin were the only persons qualified to partake in baptism. At the same time he asserted that rebaptism had no support in scripture. The Anabaptists raised the objection that Christ did not baptise children, and so Christians, likewise, should not baptise their children. Zwingli responded by noting that kind of argument would imply women should not participate in communion because there were no women at the last supper. Although there was no commandment to baptise children specifically, the need for baptism was clearly stated in scripture. In a separate discussion on original sin, Zwingli denies original guilt. He refers to I Corinthians 7:12–14 which states that the children of one Christian parent are holy and thus they are counted among the sons of God. Infants should be baptised because there is only one church and one baptism, not a partial church and partial baptism.

The first part of the document, A Reply to Hubmaier, is an attack on Balthasar Hubmaier's position on baptism. The second part where Zwingli defends his own views demonstrates further development in his doctrine of baptism. Rather than baptism being simply a pledge, he describes baptism as a sign of our covenant with God. Furthermore, he associates this covenant with the covenant that God made with Abraham. As circumcision was the sign of God's covenant with Abraham, baptism was the sign of his covenant with Christians. In A Refutation, he states,

The children of Christians are no less sons of God than the parents, just as in the Old Testament. Hence, since they are sons of God, who will forbid this baptism? Circumcision among the ancients ... was the same as baptism with us.Huldreich Zwinglis Samtliche Werke, Vol. VI i 48.10–15, as quoted in

His later writings show no change in his fundamental positions. Other elements in Zwingli's theology would lead him to deny that baptism is a means of grace or that it is necessary for salvation. His defence of infant baptism was not only a matter of church politics, but was clearly related to the whole of his theology and his profound sense of unity of the church.

Eucharist

The Eucharist was a key center of controversy in the Reformation as it not only focused differences between the reformers and the church but also between themselves. For Zwingli it was a 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 Zwingli's eucharistic theology was 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".. For more information, see . 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 affected 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.Huldreich Zwinglis Samtliche Werke, Vol. V, 613.12–13, 722.3–5, 723.1–2, as quoted in 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 his position, but it did produce some further developments in Zwingli's views. For example, he 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: