BWV 79 ("God is a light and a shield") is one of two cantatas Bach composed for the Reformation Festival, an important event in the Lutheran calendar celebrated annually on October 31. It dates from 1725 and, although less well-known than its companion, BWV 80, "Ein fest Burg," it is a work whose splendor befits the ceremonial occasion. It is lavishly scored for two horns, timpani, two flutes (added for a later performance which took place at some time between 1728 and 1731), two oboes, strings, and continuo in addition to the normal four-part chorus and vocal soloists. The text is anonymous, but may be the work of Erdmann Neumeister, an important figure in the development of the Lutheran cantata.

The cantata opens with a resplendent sinfonia; its pounding drum beats and obbligato horn parts lead directly into the choral entry, a setting of a verse from Psalm 84. The second part of this magnificent opening chorus is a dynamic four-part fugue. The alto aria which follows is a paraphrase of the same psalm verse, but the triumphant mood of the chorus is here replaced one of devotion. The third movement is an elaborate chorale setting of the famous hymn "Nun danket alle Gott" by Martin Rinkart (1636); both the timpani and the horn join their voices with that of the chorus. The succeeding bass recitative introduces an animated duet for the soprano and bass soloists calling on God to maintain his support. The cantata concludes with a simple four-part choral harmonization of the hymn "Nun lasst uns Gott, dem Herrn" by Gustav Helmbold. Three of the movements were later parodied in Bach's Lutheran Masses—BWV 234 (the alto aria) and BWV 236 (the opening chorus and the duet).

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