While the Mass is generally considered to be exclusively part of the Roman Catholic liturgy, it is also very much the basis for the Lutheran liturgy. When he broke from the Catholic Church in 1521, Martin Luther had numerous concerns about Catholic policies, but not with the liturgical practices.
Thus, in 1523 when Luther produced a document describing the liturgy of the newly founded Lutheran church, he modeled it extensively on the Catholic liturgy with which he and his followers were familiar. And Luther specified that five familiar sections (Ordinaries; see below) should still be used in the original Latin, while the rest of the liturgy would now be translated into the language of the people in the congregation: German. (Incidentally, three years later, Luther made further changes to this original Lutheran liturgy; at this time, he eliminated some of the Latin pieces altogether, and the rest he translated into German hymns.) Luther's main idea in developing the new liturgical practices was adaptability: composers and clergymen could use the Latin or the German, use more complicated arrangements for the choir or simpler versions for the congregation to sing, based on what they felt best reflected the needs and strengths of their congregation.
Many different prayers, musical items, responses, Biblical readings, and texts comprise the liturgy known as the mass. These different components fall into one of two categories: the Ordinary and the Proper. The Ordinary are those texts (which might be sung or spoken) which remain the same everyday throughout the year. The Proper are those texts which change daily. For example on Christmas Day, the entrance hymn (a Proper) sings about the birth of Jesus, but this would not be used on Easter, which focuses on Jesus' resurrections.
Composers who choose to set to music part of the Mass generally use the Ordinary. Since the texts are used everyday, the composer's music could then be used everyday as well. (With a musical setting of a Proper, a composer might hear his or her work only once every three years!)
The Mass in B Minor is the only musical setting of the Latin Ordinary that Bach wrote. In his Mass in B Minor, Bach sets to music the traditional five texts of the Ordinary that composers had focused upon for centuries. These are the Kyrie (in English, "Lord, have mercy"), Gloria ("Glory to God in the highest"), Credo ("I believe in one God"), Sanctus ("Holy, holy, holy Lord"), and Agnus Dei ("Lamb of God"). Bach's setting is a little unusual in that he separates the final section of text of the Sanctus (beginning with the word "benedictus" and lumps it together with the final movement, the Agnus Dei.)
In our day, most choirs perform and/or record the entire Mass in B Minor. But there is no evidence that Bach ever arranged for a complete performance of all the movements. Instead, movements were performed individually or in smaller groups at different times and places. This really is no surprise, since the movements were written at different times, rather than as one large collection. The idea of a recital or public concert is a fairly recent one, and sacred music (music for church) would never have been performed outside of a church service in Bach's day. So when you think about it, the fact that the Mass in B Minor was not performed in its entirety in Bach's time makes sense - these five musical pieces alone take almost two hours to perform, and that doesn't include all the additional prayers, readings, and the Propers. A complete performance as part of the Catholic or Lutheran service could take well more than three hours!
Recent discoveries and work in Bach scholarship (during the 1990's - that's actually pretty recent) now leads us to believe that, in fact, the Mass in B Minor was the last composition Bach worked on before he died, displacing The Art of the Fugue (Kunst der Fuge) from that claim; furthermore, scholars now think that Bach might have been considering the five separates movements as part of the single entity, which we know call the Mass in B Minor.
Follow the links below for more information about each section of the Mass: