File:BWV 8 1 soprano.jpg

Authorship
The cantata BWV 8 is described pp. 550–553 in The Cantatas of J. S. Bach: With Their Librettos in German-English Parallel Text, Richard D. P. Jones's revised English-language version of Alfred Dürr's 1992 German-language book on Bach's cantatas.

Neumann
Caspar Neumann is the author of the text of "Liebster Gott, wann werd ich sterben", a German Lutheran hymn in five stanzas. According to Dürr & Jones, the text of the first movement of BWV 8 is retained "word for word" from Neumann's hymn. Thus Neumann is the author of the lyrics pictured in the image above.

Vetter
Daniel Vetter published his setting of Neumann's hymn in 1713, in the second volume of his Musicalische Kirch- und Hauß-Ergötzlichkeit. According to Dürr & Jones, Vetter's chorale melody is sung by the soprano in the first movement of the BWV 8 cantata. Thus Vetter composed the melody pictured in the image above.

Bach
Johann Sebastian Bach is of course the composer of the cantata, but that doesn't mean he didn't adopt works by others in his composition, as explained in the Neumann and Vetter sections above.