The soprano duet with violin obbligato in the (*) Christe eleison second section of one of his works is in D major; that work included many operatic elements in its Credo and Gloria sections before closing on a D major chord in its Agnus Dei.
In the first part of one work by this composer, the chorales "O Mensch, bewein dein Sunde gross" and "O Lamm Gottes, unschuldig" serve as the cantus firmus. Another of this composer's works opens with a lower mordent on A, followed by descending 64th notes, an accidental C sharp 32nd, and a 16th on the tonic, all of which is octave doubled. The soprano duet with violin obbligato in the (*) Christe eleison second section of one of his works is in D major; that work included many operatic elements in its Credo and Gloria sections before closing on a D major chord in its Agnus Dei. This composer of St Matthew Passion also paired a D minor Toccata with another form and wrote six works for a certain German margrave. For 10 points, name this composer of a Mass in B minor, the Brandenberg Concertos, and a lot of fugues.
In the first part of one work by this composer, the chorales "O Mensch, bewein dein Sunde gross" and "O Lamm Gottes, unschuldig" serve as the cantus firmus.
Johann Sebastian Bach [prompt on partial answer]