How did Hesse-Kassel become Calvinist?

Hesse-Kassel became Calvinist under the successors of Landgrave William IV, who had initially established the landgraviate following the partition of Hesse in 1567.1

While the original Reformation in Hesse was carried out by Philip the Magnanimous on the model of Lutheran Saxony, the transition to Calvinism occurred later under subsequent rulers. This shift is notable because the Peace of Augsburg (1555) had officially recognized only Catholicism and Lutheranism, explicitly excluding Calvinism from legal protection within the empire. By adopting Calvinism, the rulers of Hesse-Kassel moved away from the Lutheran orthodoxy established by Philip, aligning the state with the Reformed tradition.213