The Mennonite Queen by Patrick E. Craig

Isabella, Princess of Poland, is raised to a life of great wealth and leisure in the Polish Royal Court, destined to marry a king. But fate or divine providence intervenes when she meets Johan Hirschberg, a young Anabaptist who works in her father’s stable. This chance meeting leads the young couple into a forbidden love. Together they flee Poland and embark on a dangerous journey that brings them to the city of Münster, Germany, where a violent revolution is taking place. When the revolution fails, they find refuge in the small parish of a troubled priest named Menno Simons. King Sigismund, Isabella’s father, pays Catholic Bishop Franz von Waldek to find the princess at all costs, and he pursues Johan and Isabella across Europe. Isabella does not know it, but if von Waldek captures her, she will have to make a choice that will change the course of European history forever.