What controversial religious practice did john Calvin preach?

John Calvin is often associated with the practice of predestination, a controversial doctrine within his theological framework. He preached that God has already chosen who will be saved and who will be damned, independent of individual actions or faith. This belief challenged the Catholic Church's teachings on free will and salvation, leading to significant theological debates and divisions within Christianity during the Reformation. Calvin's ideas shaped the development of Reformed theology and influenced various Protestant movements.