Protestantism
Contrary to how the Protestant Reformers were often characterized, the concept of a catholic or universal Church was not brushed aside during the Protestant Reformation. On the contrary, the visible unity of the catholic or universal church was seen by the Protestant reformers as an important and essential doctrine of the Reformation. The Magisterial reformers, such as Martin Luther, John Calvin, and Huldrych Zwingli, believed that they were reforming the Roman Catholic Church, which they viewed as having become corrupted. Each of them took very seriously the charges of schism and innovation, denying these charges and maintaining that it was the Roman Catholic Church that had left them. In order to justify their departure from the Roman Catholic Church, Protestants often posited a new argument, saying that there was no real visible Church with divine authority, only a spiritual, invisible, and hidden church—this notion began in the early days of the Protestant Reformation.

Q: When did the idea of a hidden church begin?
in the early days of the Protestant Reformation