Calvinism originated with the Reformation in Switzerland when Huldrych Zwingli began preaching what would become the first form of the Reformed doctrine in Zürich in 1519. Zwingli and John Oecolampadius became embroiled in conflict over the real presence of Christ in the Eucharist with Martin Luther, leading to a division between Lutheran and Reformed Protestants. Several theologians succeeded Zwingli, the best known of which is John Calvin in Geneva, but other reformers like John Oecolampadius, Heinrich Bullinger, Peter Martyr Vermigli, and Wolfgang Musculus were also very influential in the development of Reformed theology.