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.

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". 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.

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.