It can easily be considered presumptuous to alter the great masterpieces of the Common Practice Period, but the Mass in C Minor by Mozart was left incomplete. The Dayton Bach Society offered a commission to finish the missing sections (the second half of the Credo and all of the Agnus Dei) in the style of Mozart which would complement the existing sections, making this project a historical and wonderful jigsaw puzzle.

There were two basic requirements to fulfill on a project such as this. First is uniformity: the inserts had to be written within Mozart's style and language. Since mind-reading is impossible, the only move was to make creative decisions with an understanding of Mozart's intent, even if he might make other choices (such as the timpani and brass parts in the Crucifixus which are in a key Mozart would not have chosen).

The second requirement is continuity: the inserts needed to fit within the architecture of the original, in terms of key, tempo, forces, motives, and proportions. Naturally, the standard text of the Catholic mass was the primary tool for both uniformity and continuity. The end result is based on conjecture and compromise. If the job is done well, the audience should be completely unaware of modern contributions; the work, after all, is still Mozart's.

This project was most humbling. The apparent ease with which Mozart composed, the inevitable and logical flow of music, the perfection of balance and proportion, the ravishing beauty of the melodies, and the seamless orchestration are all great mysteries, and are far more interesting than supposition of what would Mozart have written had he finished the mass. The more listening, performing, and imitating was done, the more unattainable the solution seemed to become. This, without doubt, is the greatest testimony to his genius.

The text:

...crucifixus etiam pro nobis sub Pontio Pilato, passus et sepultus est; et resurrexit tertia die secundum Scripturas; et ascendit in caelum, sedet ad dexteram Patris; et iterum venturus est cum gloria iudicare vivos et mortuos; cuius regni non erit finis;

Et in Spiritum Sanctum, Dominum et vivificantem: qui ex Patre Filioque procedit; qui cum Patre et Filio simul adoratur et conglorificatur; qui locutus est per Prophetas;

Et unam sanctam catholicam et apostolicam Ecclesiam.

Confiteor unum baptisma in remissionem peccatorum; et expecto resurrectionem mortuorum et vitam venturi saeculi. Amen.

Agnus Dei, qui tollis peccata mundi, miserere nobis.
Agnus Dei, qui tollis peccata mundi, miserere nobis.
Agnus Dei, qui tollis peccata mundi, dona nobis pacem.

There are several other completions that have been composed:

• Helmut Eder
• H. C. Robbins Landon
• Robert Levin
• Richard Maunder
• Philip Wilby