Although Bach's Mass in B Minor is revered for its overwhelming dramatic sweep and sense of unity, it was not originally created as a single work.  It is made up of music composed over a 25-year period, some of it adapted, some of it new. Its manuscript is divided into four large sections with no overall title, and it came to be called the Mass in B Minor only by later generations.  It was not performed complete until 1859, more than a century after Bach's death.

The opening music, consisting of a Kyrie and Gloria, dates from 1733, when Bach presented it to the new Elector of Saxony, Frederick Augustus II.  It was, he said, "a modest example of the learning I have acquired in music."  In that original form with only a Kyrie and Gloria, the work was a complete missa brevis of the type that was common not only in Lutheran practice but also in some Catholic areas, including at the Elector's court in Dresden.  (No other mass by Bach has more than those two sections.)

It was not until the late 1740's, near the end of his life, that Bach began to expand this work, already his largest and most complex mass, into a full Catholic mass.  Why he did so has been the subject of much discussion.  The work as we now have it is too large to be used in a normal church service.  While some sections could have been useful in services at Bach's own church, the work was completed so late in his life -- indeed at a point when he was ill and no longer actively supplying new music -- that he may not have had a practical purpose in mind.  The most convincing reason may well be that, toward the end of his life, Bach wished to gather and preserve many of his finest works for the church by assembling them into a collection, much like other late collections, such as The Art of  the Fugue and the third part of the Clavierübung.  In assembling his music into a complete Latin mass, Bach turned to a form with a classic tradition and a sense of permanence, one that transcended the tastes of his day and the specific practices of his own denomination.

To complete the mass, he needed to add a Credo (Symbolum Nicenum), a Sanctus, and a final section comprising the Osanna, Benedictus, Agnus Dei, and Dona nobis pacem.  Most of the movements in these sections are reworkings of music that he had written earlier.  For some, he borrowed and revised (often extensively) music from his own cantatas; in a few movements, he appears to have added choral parts to what may have been instrumental concertos that are now lost.  The Sanctus, however, was originally a piece on its own, written back in 1724.  Bach's decision to adapt these movements and include them in his Mass has doubtless not only given them a wider audience but, in some cases, may have saved them from being lost entirely.

Because the music is drawn from various sources, different sections sometimes require different performing forces.  Most of the choruses are in either five or four voices.  The Sanctus is the only one to call for a six-part chorus, as well as for a third oboe, and the final section (Osanna through Dona nobis pacem) calls for eight voices divided into two four-voice choruses.  Yet despite these differences, the Mass has a compelling feeling of unity because of the care Bach has taken in structuring the whole.

The only performances of this work that are known for certain to have taken place in the eighteenth century are of separate sections:  Bach's own performance of the Sanctus on Christmas Day of 1724 and a performance of the Credo by his son, Carl Philipp Emanuel, in 1786.  C.P.E. Bach did refer to the work as a complete Catholic mass, but the first complete performance did not take place until 1859, more than a century after Johann Sebastian's death. 

Martin Pearlman, Founding Music Director, Boston Baroque
(More information can be found at baroque.boston/js-bach-mass-b-minor)