he idea of writing an arrangement of Handel’s Messiah was not Mozart’s. He was in fact commissioned to do this by Baron Gottfried van Swieten. Van Swieten had founded the "Society of Associates" (Gesellschaft der Associierten) in Vienna, an exclusive circle that organised private performances of oratorios during Lent and at Christmas. Because of the reforms introduced by Emperor Joseph II, church music had suffered from drastic changes to the liturgy that had almost brought about its total demise. For this reason, the emphasis shifted to private performances. The Viennese aristocracy was part of van Swieten’s circle and its members also acted as patrons. For quite some time before he worked on the Messiah, Mozart been part of these concerts he played cembalo under the direction of the court theatre composer, Starzer, who had already arranged Judas Maccabaeus. During this period, Mozart had access to van Swieten’s private library and was able to study scores by Bach and Handel, which he found deeply stimulating for his own creative work. In 1788 Mozart himself took over as director of these private concerts. In that same year he arranged Handel’s Acis and Galatea, then in March 1979 the Messiah, and in the following year, the Ode for St. Cecilia and Alexander’s Feast. The rehearsals for the Messiah took place in van Swieten’s apartments. The oratorio was first performed in Count Johann Esterhazy’s palais on 6th March 1789. The number of instrumentalists involved is not known, and there were supposedly only 12 singers in the choir.
Baron van Swieten, who was a great admirer of baroque music, wanted Mozart to "modernise" the oratorio. This was a perfectly normal demand the original work and its composer still commanded great respect, of course, but this was no obstacle to updating something "old-fashioned" to bring it into line with modern taste. Mozart based his arrangement on the first edition of Handel’s score. From this, two copyists produced a working score. For the English libretto and the wind sections of the original, they substituted blank lines so that Mozart could write his own accompaniment and insert the text written by van Swieten. The latter was, in turn, based on the German translation done by F. G. Klopstock and C. D. Ebeling in 1775.
The biggest change was made to the airs, as they were believed to be the form most in need of "modernisation". Mozart in part changed the harmony structure, made cuts, varied the tempi, transposed the airs or assigned them to other vocal parts. Yet he retained the form of the air with one exception. "If God be for us" (CD II, No. 23) appears in Mozart’s version as a recitative, not as an air. Van Swieten comments: "Your idea of turning the text of the cold air into a recitative is splendid... Anyone who is able to clothe Handel with such solemnity and taste that he pleases the fashion-conscious fops on the one hand, while on the other hand still continuing to show himself superior, is a person who senses Handel’s worth, who understands him, has found the source of his expression and who can and will draw inspiration from it. The mood of this "cold air" obviously had so little appeal for Mozart that he felt this was the one instance where he had to alter the formal structure, which in itself speaks volumes for his sensitivity in dealing with the original.
The choral sections remain almost unchanged. But here, however, Mozart introduces harmony. Woodwinds are added to the horn and trumpet sections and accompany the choral descant parts in unison. The trombones, on the other hand, are given the option of doubling the alto, tenor and base parts and precise stipulations are only made for two of the numbers. Before this version of the Messiah score first appeared in print, Rochlitz made the flowing comments in the music periodical, "Allgemeine Musikalische Zeitung": "He has exercised the greatest delicacy by touching nothing that transcends the style of his time ... The choral sections are left as Handel wrote them and are only amplified cautiously now and again by wind instruments."
One other change was made to the choral sections and it had to do with tempo. Mozart intervenes here, usually choosing a slower pace. In addition to slowing the movements down, he also "steals" some pieces from the choir. This applies in particular to certain virtuoso segments in the initial choral sections, which he gives to the soloists. Apart from the explanation that Mozart was doing this to illustrate baroque dynamics, this might also have been done for other reasons. It is quite possible that Mozart had no choir available whom he thought capable of performing these pieces. The airs were also shortened. For example, he cut the middle section of the bass air, "The trumpet shall sound". Of this Rochlitz wrote: "Those [airs], where Handel adhered more strictly to the conventions of his day, have been given a new and unparalleled accompaniment, one that Handel himself would have wanted, but which also incorporates the advances in instruments and taste made since his days; where the airs were too long or became unimportant, like the second part, for example, which was only written for voice and bass, such parts have been cut." Yet in comparison to other contemporary oratorio arrangements, Mozart’s cuts are minimal. They are aimed more at condensing and tightening up what is taking place. As a result, a performance of this arrangement only takes 2 ½ hours, a cut of almost half an hour. Rochlitz is of the opinion that this makes the oratorio "highly enjoyable for any kind of audience."
However, Mozart is not content with changes that are dull or conventional. He puts woodwinds into the airs to better interpret the basic mood. What’s more, he divests the bassoons of their bass function repeatedly. To preserve the musical flow of an air, he provides the singer with instrumental support in cadences instead of giving him or her the freedom to improvise. And over and above having to adhere to the rules imposed by the contemporary conventions of good taste, Mozart also had to take other circumstances into consideration. For example, in his arrangement he cut out the organ there was simply no organ available in the Viennese palais where the private performances were held. Another problem that Mozart had to contend with was the change that had taken place in trumpet playing between the time of Handel’s Messiah and Mozart’s arrangement of it. The break-up of the social order in the towns had led to the demise of the town piper guilds and, in turn, to the decline in the art of playing the clarion. The trumpets in a classical orchestra were not nearly as powerful as their predecessors, so in order to support the sound of the orchestra, Mozart "downgraded" them with regard to both harmony and rhythm. He modified the original passages or assigned them to other instruments such as the horn in the air "The trumpet shall sound" (CD II, No. 20), thus achieving a more virtuoso effect.
Yet the Messiah remains the work of Handel, despite the Mozart arrangement. Mozart did not write a new composition, he used the original as a template and arranged it or to use a present-day idiom, he did a "cover version". In doing so, he achieved a synthesis of baroque counterpoint and classical style, which is why this version of the Messiah definitely offers a remarkable alternative to the "original".