The most important early reformer other than Luther was the Swiss humanist, priest, and admirer of Erasmus, Ulrich Zwingli (ZWIHNG-lee) (1484–1531). Zwingli was convinced that Christian life rested on the Scriptures, which were the pure words of God and the sole basis of religious truth. He went on to attack indulgences, the Mass, the institution of monasticism, and clerical celibacy. In his gradual reform of the church in Zurich, he had the strong support of the city authorities, who had long resented the privileges of the clergy.

While they had their differences, Luther, Zwingli, and other early Protestants agreed on many things. First, how is a person to be saved? Traditional Catholic teaching held that salvation is achieved by both faith and good works. Protestants held that salvation comes by faith alone, regardless of good works or the sacraments. God, not people, initiates salvation. Second, where does religious authority reside? Christian doctrine had long maintained that authority rests both in the Bible and in the traditional teaching of the church. For Protestants, authority rested in the Bible alone. For a doctrine or issue to be valid, it had to have a scriptural basis. Because of this scriptural basis, most Protestants rejected Catholic teachings about the sacraments — the rituals that the church had defined as imparting God’s benefits on the believer (see "Saints and Sacraments" in Chapter 10) — holding that only baptism and the Eucharist have scriptural support.

Third, what is the church? Protestants held that the church is a spiritual priesthood of all believers, which differed markedly from the Roman Catholic practice of a hierarchical clerical institution headed by the pope in Rome. Fourth, what is the highest form of Christian life? The medieval church had stressed the superiority of the monastic and religious life over the secular. Protestants disagreed and argued that every person should serve God in his or her individual calling.