While Lutheranism was largely confined to parts of Germany and to Scandinavia, Calvinism spread into England, Scotland, France, the Netherlands, the English-speaking colonies of North America, and parts of Germany and central Europe. This expansion began during Calvin's lifetime and was encouraged by him. Religious refugees poured into Geneva, especially from France during the 1550s as the French government became increasingly intolerant but also from England, Scotland, Italy, and other parts of Europe into which Calvinism had spread. Calvin welcomed them, trained many of them as ministers, sent them back to their countries of origin to spread the Gospel, and then supported them with letters of encouragement and advice. Geneva thus became the centre of an international movement and a model for churches elsewhere. John Knox (Knox, John), the Calvinist leader of Scotland, described Geneva as “the most perfect school of Christ that ever was on the earth since the days of the Apostles.”

Calvinism was immediately popular and was appealing across geographic and social boundaries. In France it was attractive primarily to the nobility and the urban upper classes, in Germany it found adherents among both burghers and princes, and in England and the Netherlands it made converts in every social group. In the Anglo-Saxon world, Calvinist notions found embodiment in English Puritanism, whose ethos proved vastly influential in North America beginning in the 17th century. It seems likely, therefore, that Calvinism's appeal was based on its ability to explain disorders of the age afflicting all classes and to provide comfort by its activism and doctrine.

It is important to note that the later history of Calvinism has often been obscured by a failure to distinguish between Calvinism as the beliefs of Calvin himself; the beliefs of his followers, who, though striving to be faithful to Calvin, modified his teachings to meet their own needs; and, more loosely, the beliefs of the Reformed tradition of Protestant Christianity, in which Calvinism proper was only one, if historically the most prominent, strand. The Reformed churches consisted originally of a group of non-Lutheran Protestant churches in towns in Switzerland and southern Germany. These churches have always been jealous of their autonomy and individuality, and Geneva was not alone among them in having a distinguished theological leadership. Huldrych Zwingli (Zwingli, Huldrych) and Heinrich Bullinger (Bullinger, Heinrich) in Zürich and Martin Bucer (Bucer, Martin) in Strasbourg were also influential throughout Europe. Their teachings, especially in England, combined with those of Calvin to shape what came to be called Calvinism.

Developments in Geneva are illustrative of the fate of Calvinism elsewhere. In 1619 they reached a climax at the Synod of Dort (Dort, Synod of) in the Netherlands, which spelled out various corollaries of predestination, as Calvin had never done, and made the doctrine central to Calvinism. Although the synod was provoked by a local controversy, it was attended by representatives of Reformed churches elsewhere and assumed universal importance.

Calvinism underwent further development as theologians, apparently dissatisfied with Calvin's loose rhetorical writing, adopted the style of Scholastic theologians and even appealed to medieval Scholastic authorities. The major Calvinist theological statement of the 17th century was the Institutio Theologiae Elencticae (1688; Institutes of Elenctic Theology) of François Turretin, chief pastor of Geneva. Although the title of his work recalled Calvin's masterpiece, the work itself bore little resemblance to the Institutes of the Christian Religion (1536); it was not published in the vernacular, and its dialectical structure followed the model of the great Summae of Thomas Aquinas and suggested Thomas's confidence in the value of human reason. The lasting significance of this shift is suggested by the fact that Turretin's work was the basic textbook in theology at the Princeton Theological Seminary in New Jersey, the most distinguished intellectual centre of American Calvinism, until the middle of the 19th century.

Historians of Calvinism have continued to debate whether these developments were essentially faithful to the beliefs of Calvin or deviations from them. In some sense they were both. Although they abandoned Calvin's humanism, there were precedents for these changes in the contrary aspects of his thought. They were untrue to Calvin, however, in rejecting his concern to balance contrary impulses. These changes, moreover, suggest the stage in the development of a movement that Max Weber (Weber, Max) called “routinization”—the stage that comes after a movement's creative beginnings and, as a kind of reaction against the disorderly freedom of individual creativity, represents the quite different values of order and regularity. It is also relevant to explaining these changes in Calvinism that they occurred during a period of singular disorder, caused among other things by a century of religious warfare, which generally produced a longing for certainty, security, and peace.