Rossini's Messa di Gloria divides the Kyrie into three portions, the first a dotted-rhythm "Kyrie eleison" for chorus in E flat minor, the second, a more lyrical E-flat major "Christe eleison" for two tenors (unusual scoring, and likely forbidden for ecclesiastical purposes), with the first minor-key section rounding out the prayer.
The "Gloria" portion takes up the vast majority of the work. It is split up into operatic-style "numbers", soprano soloists alternating with tenors, basses, etc. The high point, emotionally, comes at the "Qui Sedes", which begins with a slow portion for chorus and tenor, then concludes with a brilliant cabaletta, showing off the extreme upper end of the tenor's range. Critics of the time were slightly scandalized by Rossini's morphing of sacred ceremony into opera seria, and even buffa, at times. Suffice it to say that this Mass sounds nothing like Mozart's Great Mass, Beethoven's Missa Solemnis, or Haydn's last twelve "name day" masses.
The concluding "number" of the Gloria was actually not composed by Rossini, but by another Italian composer more versed in counterpoint, Pietro Raimondi. Rossini wanted the work to conclude with a strong fugal section, and Raimondi provides it in a four-part double fugue setting of "Cum spirito sanctu."
Rossini's own contrapuntal abilities were displayed to even better effect in his Stabat Mater, composed for Paris in 1845. Why he felt he needed Raimondi for this work is not clear.