Baron Wilhelm Emmanuel von Ketteler (25 December 1811 – 13 July 1877) was a German theologian and politician who served as Bishop of Mainz.

He was selected over Professor Leopold Schmidt, of Gießen, whose liberal sentiments were not in line with the current Papal beliefs.

[citation needed] He is cited in Pope Benedict's encyclical Deus caritas est for his role in the Catholic social tradition.

In 1861, Ketteler published a book on reconciliation between Catholics and Protestants in Germany, Freiheit, Autorität, und Kirche; in it, he proposed the founding of a prayer society "for the Reunion of Christendom".

[3] Ketteler was friends with Julie von Massow, a Lutheran woman from Prussian nobility, who established one of these prayer societies.