This religious solomotet was composed at the time Mozart was staying in Milan during the production of his opera Lucio Silla which was being performed in the Teatro Regio Ducal in Milan. The motet was written for the castratoVenanzio Rauzzini, who was singing the part of the primo uomo Cecilio in Lucio Silla the previous year. While waiting for the end of the run (from 26 December 1772 to 25 January 25 1773), Mozart composed the motet for his singer, whose technical excellence he admired. Its first performance took place at the Theatine Church on 17 January 1773, while Rauzzini was still singing in Mozart's opera at night. Mozart made some revisions around 1780. In modern times, the motet is usually sung by a soprano.
Although nominally for liturgical use, the motet has many features in common with Mozart's concert arias, such as those drawn from his operas. Mozart also used elements of concerto form in this motet.
^K. Kuster, M. Whittall Mozart: A Musical Biography, Oxford University Press, p. 25