Two important questions hang over any consideration of Mozart’s Mass in C minor: how did it come to have so huge a design—so very different from his other masses; and why didn’t he finish it? Prior to 1781, Mozart had been court organist and concertmaster, assisting his father Leopold, who served as deputy Kapellmeister for the Prince-Archbishop of Salzburg, Hieronymous Colloredo. In this capacity, Mozart composed a number of settings of the mass for liturgical performance at the Salzburg Cathedral. The Austro-Hungarian Emperor Joseph II sought to reform the Catholic mass service, and called for sacred music with short and relatively unadorned choral passages, no aria-like solos, and no choral fugues. Colloredo followed the emperor’s lead, requiring that a mass not exceed forty-five minutes in length; as a result, Mozart's mass settings for Salzburg are relatively short, and most are not awfully interesting. For this and many other reasons, Mozart felt exploited and imprisoned, and sought every opportunity to leave Salzburg. Finally, in May of 1781, he left the archbishop’s service and moved permanently to Vienna, embarking on a career as a freelance composer. His rejection of the secure life of Kapellmeister chosen for him by his father, together with his courtship of the young soprano Constanze Weber, of whom his father disapproved, caused a rift between father and son which was never really healed; when Wolfgang and Constanze were married, in August 1782, it was without Leopold’s presence or blessing.
Shortly after Mozart's arrival in Vienna in 1781, he met Baron Gottfried van Swieten, a patron of the arts who actively promoted the music of past masters such as Handel and J.S. Bach. He owned a copy of the complete score of Bach's Mass in B minor, probably obtained from Bach's son Carl Philip Emmanuel. It is suggested that Mozart composed his Mass in C minor in reaction to his exposure to this work, and to Bach and Handel in general. He adopts the Italian, “cantata-style” structure of Bach’s Mass, dividing the texts of the five mass sections into smaller movements, each of which stands on its own, as opposed to the "through-composed" style where the entire text is sung from start to finish, without a break. Movements often alternate between elaborate contrapuntal choral settings and operatic arias for one or more soloists. The style was more operatic and theatrical than that allowed by the Emperor and the Prince-Archbishop, and resulted in a far longer mass than was allowed. Mozart imitates Handel's dotted, French overture rhythms (the signal in Baroque France for the arrival of the king) in texts addressing the King of Kings. Mozart's Mass in C minor and Bach's Mass in B minor both have an impractical combination of choral voicings, with five-part choir for modern-style choral writing, four-part choir for fugal movements, and double choir for the Osanna The orchestral layout is similarly impractical, each work containing a solo instrument that sits quietly for an hour before accompanying a single aria. Mozart's setting has a number of movements where the bass section descends, half step by half step, over the distance of a fourth, as does the Credo movement in Bach’s Mass. And the fugues are among Mozart's first efforts to compose counterpoint with anything like Bachian complexity. In discovering and reacting to Bach and Handel, Mozart adopts the grandness of their conception, leaving behind forever the relative modesty and self-effacement previously forced upon him by circumstances in Salzburg.
There is no evidence that Mozart received a commission to compose his Mass, and it is not clear why he decided to do so. He mentions the work once in a letter to his father, in which he promises a mass if he can bring his wife Constanze home to Salzburg to meet her husband’s family. It has been suggested that this Mass may have been intended as a peace offering to Mozart’s father, as an act of thanksgiving for the birth of his first child, and as an opportunity to show off his wife’s talent (she sang one of the soprano solos at the work's one and only performance). Constanze herself recalled that he promised to write a mass after her recovery from the birth of their first child.
Wolfgang and Constanze arrived in Salzburg at the end of July, 1783, with only half of the work completed: the Kyrie and Gloria movements. Once in Salzburg, he worked on the first half of the Credo and the Sanctus. Some movements are not complete in copies that have come down to us, but a number of scholars have reconstructed them. Mozart never composed music for the second half of the Credo or any of the Agnus Dei. The Mass in C minor was performed in St. Peter’s Benedictine Abbey, just outside of Salzburg (and marginally outside the jurisdiction of the Prince-Archbishop) on Sunday, October 26, 1783, in a liturgical setting; presumably the missing movements were replaced with movements from Mozart’s earlier masses, or with plainchant.
And that was the end. Mozart never returned to his Mass, except to adapt some of its music for another work, Davide Penitente, in later years. Like Bach’s Mass in B minor, which preceded it, and Beethoven’s Missa Solemnis, which followed it, the Mass in C minor would, if completed, have been far too long for practical liturgical use; one has to assume that Mozart was too busy and pressed to devote time to completing a work which stood little chance of being performed. Unlike the other two composers, Mozart became more and more committed to opera, which took up much of his time and attention for the remaining years of his career.