Brother Matthius was a German Jesuit, who despite the British hostility toward Germans in the post-World War I era, was based in England and became well regarded in British university circles.
Brother Matthius was a student of the life and works of Abbess Hildegard of Bingen and while at Oxford in July 1920, was introduced to Henry Jones, Sr. by Marcus Brody. He told Jones about a manuscript written by the Abbess, containing a vision she received of the Holy Grail, which was kept in an abbey library in Saint Gallen, Switzerland. Jones later traveled to that site to copy the relevant portions of the document.
Behind the scenes[]
Matthius can be a reference to John Matthews, the author of The Grail: Quest for the Eternal; text from this book was used to fill the Grail Diary movie prop.