The Protestant Reformation in Switzerland, initiated in the 1520s by Huldrych Zwingli, brought profound religious, political, and social changes. Zwingli, a priest in Zürich, criticized church corruption, indulgences, and mercenary practices, gaining the support of city leaders, entrepreneurs, and guilds. Zürich officially converted to Protestantism in 1523, leading to the secularization of church properties and new social reforms. Other cities like Bern, Basel, and St. Gallen soon followed, while the French-speaking city of Geneva adopted Calvinism in 1536 under John Calvin’s leadership.
The spread of Protestantism, however, divided the Old Swiss Confederacy. Several rural cantons—Uri, Schwyz, Unterwalden, Lucerne, and Zug—remained Catholic, partly due to their economic reliance on mercenary service, which the reformers condemned. These Catholic cantons formed the "League of the Five Cantons" to resist the Reformation, leading to conflict with Protestant regions.
The religious tensions escalated into two wars. The First War of Kappel in 1529 ended without major bloodshed, but the Second War of Kappel in 1531 resulted in a Catholic victory and the death of Zwingli on the battlefield. The resulting peace allowed each canton to choose its religion, following the principle of Cuius regio, eius religio. The Catholic cantons maintained control in key areas, while Protestantism took hold in urban centers and their subject territories.
John Calvin’s influence in Geneva strengthened Protestantism, spreading it across Europe through networks of scholars and refugees. Heinrich Bullinger, Zwingli’s successor in Zürich, worked with Calvin to align Swiss Protestant factions, culminating in the Consensus Tigurinus of 1549 and the Confessio Helvetica (1566), establishing the theological foundation of Reformed Protestantism.
Meanwhile, the Catholic cantons launched a Counter-Reformation to curb Protestant influence, cooperating with Jesuits and Capuchins to re-Catholicize contested areas. In 1597, religious tensions led to the peaceful division of Appenzell into Protestant Ausserrhoden and Catholic Innerrhoden. The religious split within the confederacy also shaped its political alliances, with Protestant cantons supporting Huguenots during the French Wars of Religion, while Catholic cantons aligned with Savoy and Spain.
Throughout the Thirty Years' War (1618–1648), Switzerland remained neutral, benefiting from mercenary contracts with multiple powers. The confederacy maintained its unity despite religious divisions, ensuring the neutrality of the alpine passes and blocking foreign military movements. The Grisons, however, were drawn into conflict, losing control of the Valtellina to Spanish forces before regaining it in 1639.
The Reformation left a lasting mark on Swiss society, dividing the confederacy along religious lines and influencing politics, education, and the economy. The Catholic and Protestant cantons coexisted uneasily, shaping Switzerland’s religious landscape for centuries to come.