Theologia Germanica, also known as Theologia Deutsch or Teutsch, or as Der Franckforter, is a mystical treatise believed to have been written in the later 14th century by an anonymous author. It was discovered and published by Martin Luther and became popular and influential in Lutheran pietistic circles. According to the introduction of the Theologia the author was a priest and a member of the Teutonic Order living in Frankfurt, Germany.
https://en.wikipedia.org/wiki/Theologia_Germanica