In Europa Universalis II, The Council of Trent is a scripted event which enables Counterreform Catholicism. It occurs sometime from 1570 through 1584. (Specifically, it occurs randomly on one of the 5000 days starting Jan 1, 1570.) In real history, there were a long series of meetings in Trent, from 1530 through 1563. The in-game event represents the impact of the doctrinal change from these meetings, and the spreading counter-reformation of the Church. See the wikipedia entries for the Council of Trent and the Counter-Reformation.