The motet as a musical form provided one of the earliest opportunities for composers to display their compositional flair.
The earliest extant examples are 12th century comprising a plainchant cantus firmus (the religious chant melody in long notes) in Latin with a more florid upper or lower part providing the musical interest. Musical interest went against the teachings of some of the Church fathers, however, and the composition of this form was highly regulated by the Church. The name is thought to derive from the French mots or 'words' at a point when words were added to the more florid/decorative part.
During the ensuing three hundred years overlapping time-signatures, complicated rhythmic structures and melodic lines of increasing difficulty became the norm and by the mid-15th and the 16th centuries, the motet had become standardised in a choral format with rigid rules proscribing the earlier experimentation.
It wasn't until the early 17th century that the form was re-born in the increasingly secularised format which became popular during the baroque period. Instruments and solo voices were employed, forms more reminiscent of opera and cantata came into common usage, and the final result was that it was difficult to differentiate between secular and sacred forms - the motet had become a sacred cantata in effect, and this sacred cantata (cantata spirituale, Kantate, Anthem) was the next phase in its logical evolution.
The motet survived in Latin as a major form in most Catholic countries (Italy, France, Spain and Austria) well into the latter half of the 18th century, but the fashion had changed sufficiently for the more modern forms to take over. However, is it not worth bearing in mind that works like Mozart's Solemn Vespers are simply a series of vari-form motets in the strictest definition?