Final answer:
The melodic source of the chorale Ein' feste Burg may be a popular secular tune, but Richard Taruskin's belief on this is not provided in the available information.
Step-by-step explanation:
The famous chorale Ein' feste Burg is one of Martin Luther's best-known hymns, and it has been suggested that its melody may have been based on a popular secular tune of the time. However, what Richard Taruskin believes regarding this assertion isn't directly mentioned in the information provided. It's a common historical note that reformers like Luther utilized recognizable melodies to spread their messages more effectively within an oral culture. Taruskin, known for his extensive work on music history including discussions on Reformation music, may have opinions about the origins of the chorale's melody, but this would specifically require a reference to his writings or statements in the OHWM reading, which isn't included here.