The first thing that strikes most people about Brahms' Requiem is its curious selection of texts. Whereas most Requiems use the standard Latin texts of the Mass for the Dead (like that of Mozart, for example) with the composer almost shaping their music to fit the preset text, Brahms carefully chose passages from Luther's German translation of the Bible (which has a similar literary significance in the German-speaking world to the King James Version in English-speaking areas) to convey the precise sentiments he wanted, giving himself control over both the text and the music. His choice of his native language, rather than the relatively neutral and universal Latin text, is significant in its own right. Although it might be seen as a nationalistic move - particularly when taken together with the work's full title of "A German Requiem", Brahms was in fact at pains to stress the opposite. The indefinite article "a" is important, making it clear that it is merely a Requiem in the German language rather than something connected particularly strongly to the German people, and the composer also stated explicitly that he would have been just as happy, if not more so, calling the piece "A Human Requiem".
The overarching message of the Requiem is indeed extremely humanistic. There are no references at all to Jesus, or to the Crucifixion; whereas most Requiems concern themselves with praying for the souls of the departed prior to judgement, Brahms' texts instead focus on comforting the bereaved they leave behind. The word "comfort" itself recurs in several movements. So unmistakeable is the Requiem's message that the religious authorities at the time attempted to rectify what they saw as its theological deficiencies. A letter from the organist at Bremen Cathedral, urging a more strictly orthodox work, was gently but firmly rejected by the composer, who appears to have hidden his personal beliefs by claiming musical necessity forced him to use some texts and not others.
What exactly those beliefs were is, frankly, anyone's guess. Brahms is on record as saying that he hated being taken for an orthodox church composer just because he wrote sacred works; yet the textual integrity of the Requiem shows that he was well-versed in study of the Bible and understood the significance of the passages he chose (and those he rejected). At any rate, at the work's premiere the religious authorities were still not convinced of the work's religious legitimacy. Just to be on the safe side, they inserted the aria "I Know That My Redeemer Liveth" from Handel's --Messiah-- between two of the movements. This piece, with its prominent references to Jesus and the Biblical promise of resurrection and eternal life, was presumably supposed to redress the balance and keep the concert within the bounds of mainstream Christianity, though at the cost of the integrity of Brahms' work.
Two motivations for the Requiem have been suggested. One school of thought holds that Brahms was moved by the insanity and subsequent death of his close friend Robert Schumann - Schumann and his equally gifted wife Clara had taken the young Brahms under their wing and helped in the promotion of his music - while another theory suggests that the death of Brahms' mother was the main inspiration. Certainly the addition of the fifth movement - with a prominent soprano soloist and a text including Isaiah 66:13 ("As a mother comforts her child, so will I comfort you") - would seem to support this view, although of course there is no reason why both tragedies should not have influenced the work's creation. Brahms' natural reticence about such matters is the main obstacle to forming a clearer picture of the work's genesis.
In common with a few other works (such as Orff's Carmina Burana, Rossini's Petite Messe Solennelle and Bernstein's Chichester Psalms), there exists an alternative performing arrangement (recordings available here) of the Brahms Requiem. In this instance the reduced version is scored for choir and piano duet, which can enable the work to be performed by smaller choirs than would be needed to compete with a full symphony orchestra - or alternatively simply reduce production costs for a choral society!