Historically, there were several major Reformation figures, with their own theologies.
Martin Luther was the first, and founded what became the Lutheran Church, which became the official church of the Scandinavian countries and parts of Northern Germany. His followers are Protestant in-game. The game also groups Anglicans (i.e. the Church of England) in here, probably because they're both liturgical and tend to defer to secular authority; Anglican theology tends to fluctuate depending on when and who you ask. Being Protestant in-game gives you +10% tax income and -10% idea cost.
John Calvin came later, and preached a somewhat different theology. Among the key differences were the nature of Communion and the doctrine of double predestination. You can look them up if you care. This family of churches, which both historically and in-game were known as the "Reformed Churches", were particularly dominant in the Netherlands, Scotland, and parts of Switzerland, Germany and France. In-game they give you +1 advisor pool and +10% trade efficiency.
