The Brahms Requiem may not have done much for the salvation of souls, but it provided the salvation for the composer's career. Brahms's star was in serious decline in the 1860s, with his publishers dropping him following a disastrous debut of his first piano concerto -- an early attempt at a large-scale work.
So, he set his sights on an even larger endeavor, using the concept, but not the form of a mass for the dead; replacing the traditional Latin text with his own choices of scripture, in German; and eschewing the usual religious dogma in favor of a focus upon the mourning and the bereft.
The irreligiousness of Ein Deutsches Requiem caused some nervousness before it was first presented, but all the worry was for naught. It was a breakthrough for Brahms, and it established him as one of the greatest composers of his time -- a rank which he holds to this day.