Short answers, pulled from the story.
A Mass in music is a composition that sets the invariable portions of the western Christian Eucharistic liturgy, collectively known as the Ordinary. These unchanging texts include the Kyrie, Gloria, Credo, Sanctus, and Agnus Dei, and most Masses set them in Latin, the sacred language of the Catholic Church's Roman Rite.
Guillaume de Machaut wrote the Messe de Nostre Dame, the Mass of Our Lady, in the 14th century. It is the first complete Mass whose composer can be named, predating the many anonymous complete Mass settings that also survive from that era.
L'homme arme is a secular song that became the most widely borrowed cantus firmus in Mass composition. More than 40 separate Mass settings have been built on that single tune, by composers including Guillaume Faugues, who wrote an entirely canonic Missa L'homme arme.
Pope Pius X issued the motu proprio Tra le sollecitudini in 1903, which forbade all percussive instruments, explicitly banned the piano, ended the alternatim practice between choir and organ, barred women from the choir, and required each Mass to be composed as an integrated whole rather than assembled from separate pieces. These regulations carry little force today, especially after the Second Vatican Council.
Mozart composed 18 Masses, including the Great Mass in C minor of 1782 and the Requiem of 1791. Joseph Haydn composed 14 Masses, among them the Nelson Mass and the Mass in Time of War.
The revival of choral celebration of Holy Communion in the Anglican Church in the late 19th century marked the beginning of liturgical settings of Mass texts in English. After a new translation of the Roman Missal was published in 2011, bishops in England and Wales permitted older settings to remain in use until Pentecost Sunday, 2014.