In this thesis attention is focused on two religious movements, Mysticism and Pietism, both of which made significant contributions toward the enrichment of the German language. The 14th century was the "golden age" of German Mysticism. For this reason special consideration is given to this period of history and to the three most prominent 14th century German mystics: Meister Eckhart, Johann Tauler, and Heinrich Seuse. In order to understand their struggle with the language a brief synopsis of mystic theology is given in particular their concept of how the unio mystica, the mystical union of the soul with God, can be realized. It is shown that in their attempts to “express the inexpressible” they employed several stylistic devices which greatly enhanced the language by expanding the vocabulary and exploiting the various possibilities of expression. Though separated by several centuries in time, Mysticism and Pietism shared a common bond. The late 17th century and early 18th century pietists were responsible for reviving the terminology of the mystics, and for helping incorporate their vocabulary into the German language. The religious lyric of Gerhard Tersteegen has been singled out as representative of the influence which pietism exerted on the German language. Tersteegen was greatly influenced by the 17th century quietists, who in turn were influenced by the mystics of the Middle Ages. His language bears a striking resemblance to that of Meister Eckhart. From the 18th century even down to the present his songs have struck a responsive chord in the hearts of German speaking people and have helped mold German religious thinking. One of the key words found repeatedly in the vocabulary of both the mystics and the pietists is the term Gelassenheit. I have, therefore, chosen this word as an example of the impact which Mysticism and Pietism have had on the language. As used by both the mystics and the pietists Gelassenheit meant “submissive acquiescence to the will of God”. In the course of time the word lost its religious connotation and acquired the meaning of “calmness, serenity, or inner tranquility” The importance of Gelassenheit is not so much the change in meaning, but the fact that it is still used in modern German thanks to the influence of the mystics and pietists.